Dropdown: ignore keydown events coming from `input`s and `textarea`s

Fixes #15084.

Wash-up of some stray dropdown examples

adding explicit aria-expanded="false" initial state (so they're
correctly announced by AT as being collapsed), and - in the case of
old-school use of <a> - role="button"

Don't remove focus outline from buttons in button groups

Fix Issue #15076.

Glyphicon tweaks

plus minor tweak to customizer's generated alerts

...to actually make it consistent with what I said in the glyphicon docs
section...

This commit adds documentation explaining the need to have an initial
.active element in carousels.

Closes #14951 by merging it.

Remove X-UA-Compatible caveat from Validators docs

Refs #15072

HTML validation: No longer relax the `X-UA-Compatible` error

The validator no longer complains about it unless the value is different from `content=IE=edge`, see https://www.w3.org/Bugs/Public/show_bug.cgi?id=27091

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