Narrow the scope to load fewer objects

I believe this could load HostPath hashes for HostPaths on other sites, not
just the relevant one. It wouldn't have updated the Hits on those sites as
the Hits are scoped to the relevant site.
More efficiently update the related hits

Rather than fetching and instantiating each Hit that should be related to this
mapping, just send a single SQL UPDATE to update them all.
Update HostPaths, not just hits on Mapping create
Merge pull request #285 from alphagov/improve_mappings_post_create_hits_hook

Improve mappings post create hits hook