Updating query_params causes refresh

When a site's significant query_params have changed, what a given Hit or
HostPath canonicalises to can change. This means that we might:
* show the wrong hits count for a mapping
* link to the wrong mapping from a hit
* not create new mappings from host_paths

Until now, we've ignored this problem. This change causes the relationship
between Mappings and HostPaths to be cleared and reconnected according to the
new canonicalisation rules. In turn, the connection to Hits is updated.
HitsMappingsRelations can be run for a single site

This will allow Site.update_hits_relations to only refresh the relevant
records, which will be particularly nice if many sites have their query_params
updated at once.
Only refresh relevant HitsMappingRelations

If several sites are being updated, this will drastically shorten running time.
Merge pull request #284 from alphagov/update_hits_relations_after_canonicalization_changes

Update hits relations after canonicalization changes