Remove stale service registries and refactor the repository registry

* Refactor repository registry: The difference is only the document type.
  Everything should now use `RepositoryRegistry.new.for_type("document_type")`
* AttachmentServiceRegistry and ServiceRegistry classes were no longer used
  since they're built in the abstract by passing in a document type (in
  `app/lib/specialist_publisher.rb`) now.
Merge pull request #309 from alphagov/remove-specific-repository-methods

Remove stale service registries and refactor the repository registry