in carousel example, replace img-circle by rounded-circle css class

grid-breakpoint fix [document]

v4 disabled navbar links

Update .table-responsive

Wrap .fade and .collapsing transition properties in $enable-transitions

Tweak modal sizes

Button toggling - trigger change event on input. Fixes #16673, fixes #17599.

Carousel - Do not prevent on keydown for input and textarea

See https://travis-ci.org/twbs/bootstrap/jobs/179010884 for build failure

Update use of $gray-lightest variable

Dropdown perf - on keyboard nav, find active item in the active dropdown rather than the whole document (#19953)

grid-breakpoint for sm is 576px
https://github.com/twbs/bootstrap/blob/v4-dev/scss/_variables.scss#L186

current document shows 544px, but it should be 576px

* Add some flexbox nav components
  - Includes .nav-justified for inline, tab, and pill nav components
  - Includes example of using the flex utils (.d- and .flex-items-) for centered nav
* redo heading hierarchy a bit

* Darken dismiss icon opacity
* Darken text on active list group items
* Darken light gray for accessibility/contrast, then update gray and dark gray to match