Get down to only one extra mapping query per section

* Use includes(:mapping) in HitsController#hits_in_period
* Generates one IN query for all the mappings in the section
* Not as good as a .joins(:mapping).includes(:mapping) but that breaks counting for category.hits.any?
  (which appears to be one rabbit hole too far for today)
Skip Mapping callbacks by default

* They're a drag on test speed
* They're only needed for a couple of specs
* use need_mapping_callbacks: true on a describe block when required