Hits and mappings, together at last

* Hits may now belong to a mapping
* Establish three-step update for this belongs_to using new table/model SitePath/site_paths:
  * site_paths are created from hits (pure SQL)
    (site_paths is the smallest set of paths for a site that we can operate against)
  * site_paths' mapping IDs are created by ActiveRecord batch on c14n'd path
  * Hits' mapping IDs are refreshed from site_paths (pure SQL)
* Steps can fail independently (not transactionally) as they're idempotent
* Current assumption that mappings source paths are immutable (though
  we may need to change SitePath.where(mapping_id: nil) to acknowledge editing, this
  will slow things down)