Glyphicon accessibility tweaks

- add aria-hidden="true" to avoid SRs unintentionally reading out the
Unicode character (see
http://www.filamentgroup.com/lab/bulletproof_icon_fonts.html)
- callout with advice on accessible icon usage
- replaced sr-only text in examples with (in the case of button) cleaner
aria-label
- additional example of icon used to convey meaning (in a
non-interactive control)

Badges: fix active styles when in non-anchor `.list-group-item`

- Rather than override and hack for IE8-10 and then IE11, scope to just Safari.
- Drops the IE hacks for a media query Webkit hack

Theme: Correctly override active dropdown items in collapsed navbars

Add #14868 to the compatibility docs

Customizer: Remove file type check from config import

Fixes #14978.

Use a Jekyll plugin instead of the `copy:configBridge` grunt task

Update grid-framework.less

Have Carousel ignore keyboard events from <input>s or <textarea>s

Center modal at all times

Deprecate diagonal tooltip classes

Fix modal backdrop overlaying the modal's scrollbar

Fix #14936: Use correct left/right and margin on tooltip arrows

Fixes #14916.