Generalise Attachment services

With the introduction of other types of documents, attachment services
need to be generalised. This involved removing the call to
specialist_document_id and allowing the document_id to instead be passed
in.
Adding services for AAIB report attachments
Allowing users to attach files to AAIB Reports

This commit adds an AAIB Reports Attachments controller and uses the
same views, with the only change being paths for partials and a switch
for the breadcrumbs in the attachment pages.

This also refactors out some steps to be a bit more generic and adds a
cucumber test for the AAIb Reports Attachments workflow.
Merge pull request #153 from alphagov/attachments-for-other-document-types

Attachments for other document types