Closes https://github.com/twbs/bootstrap/issues/21635

* Close dropdown menu when focusing an outside element
* Update unit test to new markup

* Remove IE compatibility mode meta tag from docs, examples, and JS tests as we no longer support IE9 and IE8
* update and remove some IE bits from our supported browser page
* update introduction.md to match
* reword starter template intro

* [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