Refactor query parsing functions in preparation for adding new focus
modes:

 - Use different sets of field names when parsing queries for the sidebar's
   filter bar and backend search. The supported filters are different.

 - Make the `focusFilters` argument to `parseFilterQuery` more general so we can
   support adding in additional filters. The immediate use case is adding in
   `cfi` and `page` filters to support new focus modes.