theme: fix invisible footnote element

Bootstrap's ["label" CSS class](http://www.tutorialspoint.com/bootstrap/bootstrap_labels.htm) is giving extra meaning to the
docutils "label" class, causing our footnote anchors to be made
invisible when there's more than one backref. Instead of making
label's meaning context-sensitive or trying to trick docutils into
applying an additional "label-info" class (which really gives us no
additional benefit and may not be possible anyway), 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.