Creating and editing  AAIB reports

This merge request allows for creation of AAIB type reports.

It contains a chunk of work to allow the SpecialistDocumentsController to deal with multiple types of documents. This should highlight what is required to create a new type of report in the system for future document types and make the process of implementing a new document type much simpler.
Ignore me, I see now that you've extracted a `Document` superclass from `CmaCase` which the new `AaibReport` also extends.