Add equal-height grid columns as an experimental example.

Based on @pete-otaqui's work from #11851
Fix carousel control margin asymmetry
Fixed typo in Responsive Tables CSS docs
Merge pull request #13284 from chasingtheflow/master

Fixed typo in Responsive Tables CSS docs
Merge pull request #13242 from killthekitten/fix-carousel-controls

Fix carousel-control margin asymmetry
grunt dist
Fixes #13208: Drop <b> for <span> in code snippets and examples
README.md: switch to SVG badges.
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