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
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
Merge pull request #13222 from twbs/fix-13126

doc how to change the collapsed mobile navbar breakpoint
move some caching info+logic out of .travis.yml; fixes #13136
Merge pull request #13269 from twbs/fix-13136

move some caching info+logic out of .travis.yml
Merge pull request #13260 from twbs/doc-12651

document that label.active on preselected inputs in btn-groups is manual
better way of tracking what caches need to be uploaded; fixes #13270
customizer: squelch error when can't save Gist
Replace $.fn.bind with $.fn.on in alert docs
Merge pull request #13272 from hnrch02/banish-bind

Replace $.fn.bind with $.fn.on in alert docs