Add gds_editor role

GDS Editors are able to see documents of all formats without belonging
to the owning organisation.
Merge pull request #156 from alphagov/add-gds-editor-role

Add gds_editor role
Refactor republish documents script and add tests

* Rerender => Republish
* Extract dependency injection to the script level, removing the need to
  inject the wiring into the DocumentRepublisher.
* Rewrite cuke to add a test around the DocumentRepublisher.
* Rewrite seed_cases method in the tests to use the apps service layer
  rather than interacting with the models directly.
* Add method to panopticon helpers to clear the received messages s.t.
  we can ensure a republish happened rather than risk picking up the
  messages from doc creation.