Fix HitsMappingsRelations so hits get mappings for the right site

* This is really slow, and we're about to fix it by making site_paths go away
Make HitsRelationsMappings.refresh! "fast"*

* Not run until heat death of universe
* Simplifies by linking hits to host_paths, not site_paths
  (109K rows vs 105K at time of writing, which seems slightly larger but means we don't have to ask
   mySQL to handle something it couldn't)