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

fixes #22007 and fixes #21976 by changing margin strategy for card deck gutters

* fix image stretching due to flexbox
* fix broke text-muted on dark bg
* no img-fluid needed