Closes #11281 by merging it (after rebasing & resolving conflicts)

Add flexbox-based equal-height grid columns as an experimental example

Fix carousel-control margin asymmetry

Fixed typo in Responsive Tables CSS docs

X-Refs:
https://github.com/twbs/bootstrap/commit/b2a67ffe55f5fedb0ae1d5dff331687c3e744be0
https://github.com/twbs/bootstrap/commit/c97e6d5cad74d20257d28734f85311ef5d00b560

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

document that label.active on preselected inputs in btn-groups is manual

move some caching info+logic out of .travis.yml

doc how to change the collapsed mobile navbar breakpoint

fixes #13216

document that you have to add .active yourself on prechecked btn-ized checkboxes+radios; fixes #12651

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.)

explicitly doc that icon classes should only be used on empty elements

Add comments in carousel.js to clarify that "slid" is not a typo