* dist

* Outline active focus (twbs#25145)

* Base the outline button :active color on the background, fixing a contrast issue

* Only apply focus outline to active when it's focused

* Add modal-dialog-centered class to vertically centered modal example.

* removes hardcoded value for nav-link padding

* Apply of twbs#25068 to input group

* dist

* docs: Fix overflow issue in IE. (twbs#25142)

* Allow nested structure for accordions (twbs#25121)

This commit allows nested structures for accordions. Also a part of
the documentation about data-children is removed because this
functionality didn't work and it's not applicable anymore.

Tests with the collapse accordion are also a bit adjusted to the new
situation.