v4 alpha 3 ship list

Didn't do one of this for the second alpha (which kinda sucked). Trying again this time with the highlight-worthy changes as we go.

Summary of changes

Updated grid system with new mixins and predefined classes.
Overhauled form controls, documentation, and more to fix bugs, reduce code bloat, improve layout options, improve validation styles, and more.
New native font stack to replace the Helvetica-Arial one for improved text rendering across all operating systems and devices.
More selector improvements that reduce HTML requirements by better utilizing classes.
Added dozens of new variables for improved customization (more still to come).
Updated lots of documentation to better explain components and more.
Massive improvements to flexbox support throughout the project, most notably on the grid.
Renamed .label to .tag to disambiguate our custom component from the HTML element.

CSS

#17194 (closed): Ensure height of select.form-control matches textual inputs.
#17222: Replace base64 PNGs for form validation icons and custom form controls with smaller, slightly customizable inline SVGs.
#17498 (closed), #18130 (closed): Removed horizontal padding on .form-control-label so it avoids overriding grid-based form layouts and renders correctly in stacked forms.
#17756: Fix flexbox column wrapping in Firefox and IE10+ by adding a max-width that equals the column width.
#18398, #18921: Add new _custom.scss file to the import stack for easier variable overrides.
#18390 (closed): Renamed .radio and .checkbox to .form-check, and .radio-inline and .checkbox-inline to .form-check-inline.
#18488: Turn .text-muted into a full utility class with hover styles.
#18500: Allow multiple instances of .input-group-addon per input group.
#18515 (closed): Added .form-control-{sm/lg} classes to match small and large input classes, namely for vertical alignment of label and input in horizontal form groups.
#18573 (closed): Renamed .form-control-label to .col-form-label for it's more contextual use case as a horizontal grid layout option.
#18695: Condition-alize the container classes around the $enable-grid-classes boolean option (joins the existing conditional .col-* classes).
#18702 (closed): Updated form validation docs to include basic examples for situations in which you'd want to use each state.
#18703 (closed): Dropped .text-help for .form-control-feedback, designed specifically to only display temporary form validation feedback text in addition to normal help text.
#18771: Rewrote custom form components (new classes, less nesting, disabled checkboxes and radios) along with some docs tweaks.
#18772: Renamed .btn-*-outline classes to .btn-outline-* to make it easier to style with attribute selectors.
#18774: Rename .card-*-outline classes to .card-outline-*.
#18782: Dropped the .pager component since it's essentially a lightly customized version of v4's new outline buttons.
#18783: Revamp a ton of nav variables for easier customization and tweak some nav docs.
#18789: Prevent breadcrumb separator from getting underlined on hover when not using <ol> markup in IE9-11 with a funky little CSS hack.
#18807: Overhaul dropdown docs and improve split button dropdowns with some fresh styles.
#18814: Added Liberation Mono to the monospace font stack for the sake of Linux users.
#18823: Add $btn-active-box-shadow variable for improved customization of button states.
#18848: Only round progress bar borders when $enable-rounded is true.
#18925: Added :lang()-based support for translating the text in the custom file input widget.
#18927: Apply word-wrap:break-word to tooltips and popovers.
#19000: Remove print-specific max-width:100%; on images so that Google Maps etc. print correctly.
#19064 (closed): Overhaul tables docs and use rgba() colors for inverse tables.
#19098: Brand new "native font stack" that prioritizes the well-designed display fonts from each major OS and device over traditional web fonts.
#19099, #20349, and #20361: Overhauled grid system to add docs for flexbox, add flexbox utility classes, improve flexbox responsive behavior, streamline mixins, and more.
#19102, a0a157db: Drop .center-block for .m-x-auto.
#19103: Revamp the {a,button}.list-group-item selector with a new class, .list-group-item-action. Includes revised docs and more code comments.
#19105: Update modals to use max-width within their media queries to prevent horizontal scrollbars.
#19106: Added $enable-print-styles variable to control whether default styles for @media print are included.
#19109: Integrated postcss-flexbugs-fixes into the build to workaround Flexbug #4 which affects IE10-11.
#19121: Refactored button and input padding and line-height for improved stability after changing root font-size.
#19136: Change from hex border colors on .form-control to rgba() with background-clip: padding-box so we have borders that blend in with their background elements.
#19139: Add support for validation states on custom forms.
#19157: Rename .label class to .tag to disambiguate between a custom component and <label>s.
955e0405: Added support for tab and pill navigations in card headers.
#19234: Add .w-100 as width: 100% utility class
#19236: Extract $carousel-icon-width variable
#19252: Add $card-img-overlay-padding variable
#19277: new .form-control-legend class
#19370: Add $input-bg-focus and $input-color-focus variables
#19411: Neutralize link styles for placeholder links/named anchors
1b351058: Add display utility classes
#19563 (closed): Add variable for background-color of .bg-inverse utility class
#19667: Use border-radius mixins and add $nav-tabs-border-radius variable
#19714: Suppress IE/Edge additional white/blue colors for focused <select>
#19735: Add disabled styling for .custom-select
#19771 (closed): Fixed form control sizing not applying to <select>s.
#19862: Update Normalize to v4.0.0
#19874: Make named anchor/placeholder link style reset more specific by targeting <a>s without an href and tabindex.
#19885: Several card improvements for spacing, contextual variants, bug fixes, and more.
#19889: Updated popovers to include a variable for the inner padding and restore CSS-powered hiding of empty popover titles with .popover-title:empty.
#20083: Add media rules to stack nav items in navbar-toggleable-*s.

JS

#19894 Update jQuery to v2.2.4

Accessibility

#19222 Focus (visually hidden) input radio/checkbox
#19255 add/unify advice on contextual colors and accessibility
#19705 Add aria-label and callout about labelling pagination components
#19879 Handle aria-hidden on modal container when showing/hiding

Docs

Miscellaneous accessibility fixes.
#17089 (closed): Improve responsive test examples.
#17248 (closed), #17266, #19277: Update docs to use <div>s over <fieldset>s in form groups as fieldsets are more appropriate for groups of multiple inputs.
#18492 (closed): Add .h1 – .h6 example to docs
#18545 (closed): Combined the "Dropdowns" and "Button dropdown" docs pages into one
#18704: Updates form validation docs to include examples of validation feedback text alongside always-present form field help text.
#18705 (closed): Improve Migration Doc for .help-block and .form-group
#18768 (closed): Add migration guidance for Dropdowns
#19277: better use of fieldset/legend for grouped controls
#19408: Introduction for using table-reflow
#19439 (closed): Include reasoning & suggested replacements for dropped widgets in migration docs
#19459: Explicitly state the ordering of cards in card columns in the docs
#19476: Added explicit commentary to .card-* classes
#19567: Call out removal of @screen-* variables in v4
#19710: Update 300ms click delay docs to reflect iOS 9.3 improvements
#19789: Suggest alternative approaches for .hidden and .show in migration docs
#19863: Add docs for inverse table "variants" using existing utilities
#19892: Remove reference to loaded.bs.modal event
#20372: Enable flexbox CSS on flexbox grid page with custom override file for hosted docs

Examples

#18741: Fix justified nav example by updating it to the new nav classes (note: it still exhibits the same bug on window resize as documented in v3).

Packaging

#19427: Add jQuery and Tether to npm dependencies
Fixed NuGet integration
#19706: Add GitHub issue template
#20141: Require Node.js >= v4 in engines in package.json