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)

* 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

* Fix bug with dropdown tab links not deactivating when other tab or dropdown link is clicked

* Revise bug fix for more stability

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Update Normalize to v5.0.0.

Fix pagination underline

switch to rgba for list-group border color

renamed modal-title-padding to modal-header-padding

Fixes #21094: Move text-decoration override from the default link state to the hover/focus state so it can override the global styles properly

Refs https://bugs.chromium.org/p/chromium/issues/detail?id=510079
Closes #21120

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