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

- add .col-*-auto to the extend in our grid framework mixins so it gets padding
- this means we can avoid the col-12 classes in our docs for the responsive variants

Change the .navbar-toggler to default contextual .navbar-*-color.

<iframe height='265' scrolling='no' title='Bootstrap - .navbar-toggler color' src='//codepen.io/zalog/embed/MJLZOE/?height=265&theme-id=light&default-tab=css,result&embed-version=2' frameborder='no' allowtransparency='true' allowfullscreen='true' style='width: 100%;'>See the Pen <a href='http://codepen.io/zalog/pen/MJLZOE/'>Bootstrap - .navbar-toggler color</a> by Catalin Zalog (<a href='http://codepen.io/zalog'>@zalog</a>) on <a href='http://codepen.io'>CodePen</a>.
</iframe>

instead of flex column, use wrap and an explicit flexbox break with flex-basis 100% on the navbar-collapse.

add some additional container styles for enabling flex behavior when content isn't immediate child of .navbar

Generate correct breakpoints for navbar-toggleable-* and generate toggleable without media query to never collapse

* Update _screen-reader.scss

Small improvements that could be important:
* `clip` [is deprecated](https://www.w3.org/TR/css-masking-1/#clip-property). Adding `clip-path` as progressive enhancement; the shorter notation came from @ryuran 's [suggestion](https://twitter.com/ryuran78/status/778943389819604992);
* [J. Renée Beach warned about single pixel with interfering with screen readers vocalisation](https://medium.com/@jessebeach/beware-smushed-off-screen-accessible-text-5952a4c2cbfe#.vcd5xlpgg) solved with `white-space`.

See [the detailed post on Hugo Giraudel's blog](http://hugogiraudel.com/2016/10/13/css-hide-and-seek/).

Also kinda related to issue #20732 :)

Please let me know if you find any trouble with this technique. Thanks a lot!

* Reorder declarations

To pass Hound.
Didn't think `clip` and `clip-path` would be considered as unrelated…

* Trailing spaces

* Last trailing space?

* Remove margin: -1px

- Justified and fill nav variants need .nav-item classes on the anchors in addition to .nav-link
- Fix broken styles for the .nav-justified class which wasn't actually doing what we needed it to

- revert back to display block for list items to prevent unwanted behaviors
- fixes a handful of other linked issues (see main thread)
- updates docs to include more utilities for the one example

<label class="custom-file"><input type="file" class="custom-file-input" /><span class="custom-file-control"></span></label>
VS
<label class="custom-file"><input type="file" class="custom-file-input" /><span class="custom-file-control">test.txt</span></label>

* Fixed some linting issues

* Run npm tasks after scss cleanup

* Revert "Run npm tasks after scss cleanup"

This reverts commit 1103a0da.

* Property sort order for grid

* Let's respest the property order in the mixins

* Respect property sort order in reboot file

* ::-ms-expand is a vendor-prefix, add it to the scss-lint disable

* Revert hover mixin comment

* Fixed missing mixin hover-focus

fixes #17573

Despite now having the ability to override outline-variant color, it's not actually being used so each are still #fff

Related: https://github.com/twbs/bootstrap/pull/20734

— Moves print display utils to display utils file
— Removes all other .hidden- utils
— Expands on .invisible and it's mixin to provide options for visibility: visible and visibility: hidden

* Drop -x and -y as they're all the same
- Also move -width to elsewhere in the vars because it makes no sense by spacers.
- Update values of -x and -y across main Sass and docs Sass.
* Update docs to reflect changes; link to spacing utils from options page