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