* [Fix #19849] Tabs are opened even if disabled.

* fix hund code review hints

* rollback hound issues because Travis fails

Fixes 2 bugs:

1. All keydowns were being prevented. Because of that the user wasn't able to navigate in the whole page using ARROW_UP/ARROW_DOWN.

2. Even when  is an input or textarea the keydowns were being prevented. Because of that the user wasn't able to type any text on these elements.

* fix wrong activation of all nested links;
just first level item should be activated

* use template instead of string concatenation

* Throw error when a plugin is in transition
* Add unit tests about plugins in transition

When the rubberband effect causes Safari to scroll past the top of the
page, the value of scrollTop becomes negative. If the offset of the first
ScrollSpy target is 0 - essentially if the target is at the top of the
page - then ScrollSpy should not clear the active item. Conceptually, the
first item should remain active when rubberbanding past the top of the
page.

This commit fixes issue #21055 by verifying the first scrollspy target is
not at the top of the page before clearing the active nav-item.

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

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

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

* Revise bug fix for more stability

* Explore responsive display utils, but with a twist: lowest breakpoint has no breakpoint modifier in the class name
* make floats use the same format, add float-none mixin