https://www.youtube.com/watch?v=L-4VxEtWyRo

Fixes #14319: Prevent jumbotron reflow by ensuring padding of .container isn't interferred with by .jumbotron side padding

Closes #13790 by merging a rebased version of it.

Fixes #14038.

Research supporting this choice of approach:
    https://gist.github.com/cvrebert/68659d0333a578d75372

Why not <button>?
Even with a `tabindex` attribute, they don't gain focus when clicked upon in:
    * Mac OS X Firefox 30.0
    * Mac OS X Safari 7.0.5

Fixes #14095: Add basic styling for captions, which we've for some reason been missing all this time.