theme: fix invisible footnote element

Bootstrap's "label" CSS class is overriding 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, that would be a trivial update.