use "topics" consistently throughout code

We really shouldn't use specific field names in the code at all...
use named URL for precoordinate facet terms, simple URL for others

Named URL works for things like topic and genre and format but
the current URL pattern is not flexible enough for fields like
location that can contain freetext.
search.term_query URL can now accept spaces, commas

Needed to support location field.
detect ID numbers (int) inside term ID strings
made models.display_facet match term_id rather than term text

In the case of something like "Publications [74]" match the "74"
bit.