Process footnotes in USX input

Any <note> elements in USX input will become <a> elements with an
appropriate footnote marker. Right now we're using lowercase Latin
letters, because that's simple. If it's a desirable feature to allow
footnote markers from other alphabets, that could probably be handled by
turning this into an Angular directive that gets an option from the
project settings, in which case the PHP UsxHelper code would become
extremely simple: just produce "<footnote marker="automatic">footnote
contents</footnote>" and let the footnote directive worry about
rendering it appropriately. (The directive could append each footnote in
turn to whatever element has the HTML ID "footnote-contents").

However, that approach was deemed too complicated for now, so we chose a
pure-PHP approach instead.