Performance of hits pt1

* 30% improvement on summary (avoid counting things)
* General, smaller improvement on everything else by avoiding N+1
  for each hit row by preloading the host and site for its row
It doesn't make a difference to performance, but it avoids the situation where a `LEFT OUTER JOIN` to `mappings` brings in a second `path_hash` column, thereby generating invalid SQL.

That said, since I removed the `eager_load` in favour of `includes` precisely to *avoid* the `LEFT OUTER JOIN` (which changed the result of the query) it's not needed now, so I'll remove it.