Add aaib reports to rummager

In order to expose AAIB Reports in search, we need to send the documents to Rummager when publishing.

This PR contains:
* adding observers for publishing and withdrawing of AAIB reports to/from Rummager along with relevant tests
* refactoring SpecialistDocument Cucumber steps to be more generic
* renaming of the routes from specialist_document to cma_case

Ticket: https://trello.com/c/X4vUOJzE/203-add-aaib-reports-into-search-results-2
I don't think this wants to be changed — the URL is the same for all specialist documents, as evidenced by you now calling `published_cma_case_path` and passing it an AAIB report.