Add support for RAIB reports

This PR contains the work in specialist-publisher for editors from the
"Rail Accident Investigation Branch" to publish reports. It contains:

 - the schema of the possible values of the facets in the RAIB Report
   form
 - the models, services, observers, repo and views for RAIB reports
 - adding RAIB editors to permission checker
 - testing RAIB Report workflow
Whilst it might be (and probably is) completely valid to remove this validator for Attachments -- i suspect it's left over from whitehall-style "attached HTML documents" -- can this be in a separate commit, with a rationale for its removal?