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.
WIP - Update to account for new schema, move test data across to fixtures so the build passes
Move import implementation to app/importers (+specs)
Add test for AaibAttachmentImportMapper

While testing, realised repo is not required. Removed it.