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
Customizer: switch to UglifyJS2 (v2.4.13).

The uglify build is generated with `uglifyjs --self -o uglify.min.js`.
Add the jquery check in customizer's output.
Add the license header in the non minified build too.
Run `grunt`.