Create DFID importer to import...DFID reports!

 * Create DFID import entrypoint, attachment mapper.
  * No mapper required.
 * Adjust the import_reports script to use ENV['IMPORT_KIND']
  * `IMPORT_KIND=DFID bin/import_reports /data/import`
  * `IMPORT_KIND=AAIB bin/import_reports /data/import/metadata /data/import`
 * Extract method refactor on AaibImport
 * Create DocumentImport::AttachmentHelpers to extract common utility
   methods for attachment mappers.
 * AAIB fixtures now live in `spec/fixtures/aaib_import`
My reasoning here behind environment variables was that an import can be triggered over and over again with the same or different data sets, but in each case it's likely (across a single session) that the kind will be the same.

Keeping it simple would suggest we create `bin/import_aaib_reports, `bin/import_dfid_reports` and remove them (with the no-longer-required code) once the import is complete.

I'll do that instead.