Rename 'admin' to 'GDS Editor'

We currently use 'admin' to mean 'can edit any site', whereas we want to add
the ability to edit things which are beyond mappings (eg the redirection host
whitelist).
Merge pull request #294 from alphagov/rename_admin_permission_to_gds_editor

Rename 'admin' to 'GDS Editor'
Add a global_type column to the sites table

- Due to doing this, make global_http_status and global_type stay in
  sync with each other by setting both attributes in previous data, with
  some manual SQL in a migration. Following @jennyd's lead with what she
  did with mappings.
- This will eventually replace global_http_status, in the unresolved
  mappings process.
Merge pull request #295 from alphagov/change_global_http_status_to_global_type

Change global_http_status to global_type
Eager load site -> hosts 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.