Merge pull request #131 from alphagov/require-manual-document-builder

Add missing require for ManualDocumentBuilder
Add AAIB report model and spec

For AAIB transation, the user must be able to create reports on air
accident investigations. This commit adds the model with the necessary
spec.
Add AAIB report artefact formatter
Add AAIB reports controller
Add AAIB report form Class
Refactor common CmaCaseForm into DocumentForm

Common code required in CmaCaseForm and AaibReportForm refactored into
its own class
Move CMA Case extra fields to their own partial

Move rendering of extra fields for document types into their own partial.
This will allow us to specify extra fields for the view per organisation.