Add hit_count to Mapping

* For precomputation to assist Sort By Hits
Micro-optimise Site#hit_total_count

* Get it in one hit, don't iterate rows
Make spec language consistent

* We only use "should" in assertions
Refactor Import::HitsMappingsRelations

* Instance it so we're not passing around site
* Remove some duplication around IN clauses
Precompute mapping hit counts

* Part of Import::HitsMappingsRelations.refresh!
* Not too onerous a query
Remove Mapping#with_hit_count

* expensive scope
* hit_count now precomputed and available as an attribute on Mapping
Merge pull request #327 from alphagov/precompute-mapping-hit-count

Precompute mapping hit count