BS3: Navbar overhaul

This overhauls our navbars to address a few items:

• Navbars are now responsive by default (fits better with mobile first approach)
• Navbars now require .nav-header to group .navbar-brand and .navbar-toggle for proper mobile display in conjunction with the collapsed content.
• Renamed .nav-collapse to .navbar-collapse for a more consistent naming convention.
• Refactored dropdown menus to use the old 2.x approach (no longer absolutely positioned, but rather as navbar nav links). Fixes #8792 (closed).
• Simplified examples in docs to only highlight content not displayed in the first navbar example.

It can add some heft to an otherwise lightweight navbar, but I think all these improvements help the component and make it more durable in the long run. Still needs some finessing, but it's close I think.