Eager load site for filtered mappings
Eager load taggings => tags for mappings

* also rephrase the _tags partial so it uses the eager-loaded collection
  over tag_list, which doesn't participate in load caching
* this means that we don't query tags once per row in the index, instead
  just referring to things we pre-loaded in the filter
* Around 70-80% improvement in filtered mapping load speed
Switch Host#excluding_aka to an indexed column

* Was using unindexed with a LIKE
Optimise User#can_edit_site?

* It won't change within a request, so cache the result on first
  access.
Switch location of scope periods in filter
Cache the default host on first hit

* Avoids deeper SQL cache hit
Cache show_preview_links? result

* Avoids requerying @site.default_host.aka_host every row
  (would still hit SQL cache, but slower than cache)