Improve facet styling

1) Facets currently display very poorly when the number reaches 3-4
digits.  Facet data is more or less a two column table, so let's style
it that way to make it more robust.

2) Rearrange some styles to reduce redundancy, and allow overflow
where possible.

3) Change the selected highlight to look a little better.

Signed-off-by: Dan Wells <dbw2@calvin.edu>
Signed-off-by: Ben Shum <bshum@biblio.org>