Add 'RAIB reports' as an artefact format for specialist-publisher

In order to transition RAIB to GOV.UK we need to add their reports as
an artefact kind.

The owning app is specialist-publisher.
Merge pull request #239 from alphagov/add-raib-report-as-artefact-kind-for-specialist-publisher

Add 'RAIB reports' as an artefact format for specialist-publisher