* Remove aria-expanded from collapse.js target element

aria-expanded="true"/aria-expanded="false" only applies to the trigger,
not the element that is being expanded/collapsed.

* Tweak collapse.js accessibility section

...to make it clearer that the aria-expanded attribute always just goes
on the control.

* Fix collapse.js unit tests

- reword some of the text to make it clear we're checking behavior of
trigger/control
- move incorrect aria-expanded out of the <div>s and to the actual
trigger/control <a>s
- fix incorrect test assertion text output false -> true

* Only change aria-pressed if it's not an input-based radio or checkbox group

aria-pressed="true"/aria-pressed="false" is really only useful for
making on/off toggles out of, say, `<button>` elements. the attribute is
useless (and potentially confusing/conflicting) on, say, `<label>`
elements for an existing `<input type="radio">` or similar.

* Add unit test for buttons.js and radio/checkbox inputs in button groups

`role="tabpanel"` is not appropriate as role for the overall container - it only applies to the individual content parts of each tab

While `placeholder` is nominally valid per spec
http://rawgit.com/w3c/html-api-map/master/index.html#accessible-name-and-description-calculation
it is inelegant, a fallback, and not supported in ios/safari/voiceover
and android/chrome/talkback, to name a few combinations

`aria-describedby` is also not really the right tool for providing a
name/label equivalent, so remove that from the list of possible
alternative methods / clarify how it can be used

Prevent default for ESC in modal.js

`role="tabpanel"` is not appropriate as role for the overall container -
it only applies to the individual content parts of each tab

ESC can be used to close modals, but on OS X/macOS this also jumps out
of full-screen mode. `preventDefault` suppresses this.

Fixes #22056 by ensuring outline cards make their header/footer transparent and use the right border color.

* no more outline overrides
* rename old .open to .show

- Drops the `if-supports-3d-transforms()` mixin since all our browsers do support it

- Updates carousel to not rely on that mixin

Revert "Drop width from sizing utils given .col- classes can do the same thing (follow up to #22376)"

This reverts commit 2f21403a.

- Pulls in values from v3
- Closes #22371 which didn't use a transition include

- Closes #21622 which kept the 3px
- Puts variable in proper order for shorthand (y x)

- Closes #21760
- No longer needed since we have an rgba() value for this element, so it naturally darkens

* tweak some copy
* fix up scrollspy docs
* remove nav styles that were added
* fix nav-based docs by requiring .nav-item on .nav-link

Order y & x axis by top, right, bottom, left logic

Vertical centering `.navbar` flex children's

Demo: http://jsbin.com/fibini

Set/reset `.dropdown` position when `.navbar-expand`.

Demo: http://jsbin.com/pojecuh

Force .container-fluid to be 100% width no matter if the parent is flex, or not.

Pen: http://codepen.io/zalog/pen/yMwyGr