Create attachments when performing an import

We need to attach the "assets" provided from the data dump to the aaib
reports as attachments, and replace their references in the body text.
Move import implementation to app/importers (+specs)
Add test for AaibAttachmentImportMapper

While testing, realised repo is not required. Removed it.
All attachments should be referenced in doc body

When importing, wrap the document in a decorator which adds validation
to ensure all attachments are referenced in the document body.

This means that a document will fail to import if all its attachments
are not identified and replaced with the correct govspeak snippet.