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 filtering by path to hits