first draft of issue sorting / display
more generic bib_headings approach

store the possible major/minor sort keys in a global, VIS.bib_keys.
We'll validate sort choices against this global. We'll also use it to
set a CSS class on each bib heading, so that we can style different
major keys differently (not a beautiful solution, but lets us flow long
sort headings differently).
more bib heading styling

and set a max height on the bib headings list, so that it has a scroll
bar rather than spilling off the screen when it gets too tall
regenerate minified source