Add maib reports

This PR contains the work in Specialist Publisher for editors from the Marine Accident Investigation Branch to publish reports. It contains:

- the schema of the possible values of the facets in the MAIB Report form
- the models, services, observers, repo and views for MAIB reports
- adding MAIB editors to permission checker
- testing MAIB Report workflow

Publishing requires https://github.com/alphagov/rummager/pull/302 and content models to be bumped in Panopticon and this.

- [ ] bump `govuk_content_models`
We really need to figure out a way to make the menubar not terrible for GDS editors. Not going to block this PR though.