Add equal-height grid columns as an experimental example.

Based on @pete-otaqui's work from #11851
Merge branch 'master' into equal-height-experiment
add screenshot
the space between spaces
Merge pull request #13203 from twbs/equal-height-experiment

Add flexbox-based equal-height grid columns as an experimental example
When opening the dropdown, focus() the dropdown-toggle BEFORE triggering 'shown.bs.dropdown'

Closes #11281 by merging it (after rebasing & resolving conflicts)
grunt dist
JS: remove unused variables.
Run `grunt dist`.
Merge pull request #13091 from twbs/unused-js

Remove unused javascript
grunt dist
Wait for the expected target during a collapse

Actually wait for the collapsed item to complete. If complete is called
with other target, instead of simply returning and do nothing, wait again
until the proper target triggers the event (otherwise this leaves collapse
in a broken state).
rm not particularly useful @zindex var comments
fix #13340 by documenting @icon-font-path & @icon-font-name more
Merge pull request #13353 from twbs/fix-13340

Document @icon-font-path & @icon-font-name more
Merge pull request #13330 from Kronuz/patch-1

Wait for the expected target during a collapse