Things missing from v4 migration docs

Created by: qaisjp

Sorry, I know multiple issues in threads are frowned upon but here are a few things that needed to be added to the docs

1. dropdowns no longer require li
2. .navbar-header no longer necessary, I think .navbar-body was also a thing (and is no longer necessary)
3. .card-danger+.card-inverse isn't exactly the same as .panel-danger
4. media objects in docs are moved from components to "layout"
5. removal of badges aren't mentioned (.badge can be directly replaced by .label .label-pill .label-default)