Add RAIB report format

This commit adds the ability for Specialist Frontend to render RAIB reports. This contains a presenter which is called from specialist_documents_controller and a features for testing viewing RAIB reports.
The trello ticket (https://trello.com/c/Mtd6YDSr/371-raib-finder-add-support-to-rummager-and-finder-api-for-raib-reports-1) says that this format should have a finder name of "Rail Accident Investigation Branch reports", so we should singularise this if that's still the case.