Multiple-select facets (such as type and language) do not work properly
on versions of Solr (at least) greater than and including 4.4. The problem
is that when selected the filtered counts are not applied to other (non-
multi-select) facets. The problem seems to be that we were simply doing things
wrong, and a Solr bug made this appear to be not the case. Changes to address
this:

 - marking a facet multi-select means that it only tags and excludes itself
   from winnowing the counts.
 - various other cleanups as a result of this simplification