Move bulk add mappings filters into a module

* Previously, we had to duplicate the same filters across controllers
* Since we're already including the module, make that include happen
  via a method call instead
* That method call passes through the options to before_filter
* Since the use of the filter also depends on find_site, move that
  into the module too
Be consistent in language with other mixins

* acts_as_taggable_on
* has_paper_trail
* tracks_mappings_progress (not the imperative track_mappings_progress)
Merge pull request #282 from alphagov/refactor_saved_mapping_ids_module

Move bulk add mappings filters into a module