Create DFID importer to import...DFID reports!

 * Create DFID import entrypoint, attachment mapper/mapper.
 * Add import_dfid_reports script and rename aaib reports script
 * 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`