Extract AaibReportServiceRegistry from services
Extract AbstractDocumentsController

Now that CMA cases and AAIB reports both use a service registry with the
same interface we can extract most of the common behaviour into a
superclass.
Merge pull request #169 from alphagov/extract-common-controller-behaviour

Extract common controller behaviour
Importer set up initially to import AAIB reports

From `import/metadata` (json files, see spec/fixtures/import) and
`import/downloads`, import AAIB reports.

Also create an optional decorator used to validate that all attachments
for a document have been referenced in the body of that document.

Worked on by myself and @bestie
Handle duplicate titles.

 * For documents which already have a newer version, just skip it.
 * For documents which have an older version, delete that version and
   import the newer instead.
Strip PDF tags from AAIB body on import