Add equal-height grid columns as an experimental example.

Based on @pete-otaqui's work from #11851
fixes #13216

change `@zindex-popover` and `@zindex-tooltip` values to be greater than
those set for `@zindex-modal` allowing tooltips and popovers to be
displayed in front of modals (and all other content).
Fix carousel control margin asymmetry
Add comments in carousel.js to clarify that "slid" is not a typo

Addresses #13248.
Merge pull request #13249 from twbs/slid-is-not-a-typo

Add comments in carousel.js to clarify that "slid" is not a typo
explicitly doc that icon classes should only be used on empty elements; closes #13240
Merge pull request #13241 from twbs/address-13240

explicitly doc that icon classes should only be used on empty elements
Update "grunt-contrib-concat" dependency.
Run `grunt`.
re-deprecate @screen-xs-min per #12913

By definition, XS screens have no minimum size, only a maximum size.
(Symmetrically, LG screens have no maximum size, hence why there's no @screen-lg-max.)
help avoid misuse of Carousel's .active when copy-pasting from examples; fix #13252
document that you have to add .active yourself on prechecked btn-ized checkboxes+radios; fixes #12651
Merge pull request #13217 from thedavidscherer/fix-13216

fixes #13216
Update dependencies.
Fix the new JSHint warnings.
Run `grunt`.
add timing logic+output to s3_cache.py
grammar
Travis: introduce $GEMDIR env var
doc how to change the collapsed mobile navbar breakpoint; fixes #13126