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`