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
Tweak JS style and the relevant JSHint and JSCS checks.
Comply to the new style.
Remove `laxbreak`.
Remove `boss`.
Enable `requireCurlyBraces`.
Enable `es3`.
Switch to using plain `if/else` statements when there's no assignment.

The minizer outputs the same code anyway, and this has better readability.
Minor whitespace changes.
Switch to `(function){..})()`.
More updates to the JSHint options.