Use is_solr_supported helper for search app

The search app alters queries based on whether Solr is supported or not,
so we should use the newly introduced helper for that. Support for
non-Solr backends is questionable at best...
Copy hack to keep facet selections when changing relevancy

SearchView just passes request data through to keep facet selections.
It's not a security risk, but still a nasty hack. But as this PR to
improve search has already grown to seach epic proportions, we just
imitate the same thing here.