theme: fix invisible footnote element

Bootstrap's "label" CSS class is giving extra meaning to the
docutils "label" class, causing our footnote anchors to be made
invisible. Instead of making label's meaning context-sensitive,
I chose to just rename Bootstrap's version of "label" to
"bootstrap-label".

To the best of my knowledge we aren't using Bootstrap's label for
anything, but if we are, it's a trivial update.