Remove validation of therapeutic_area from DSU

This field isn't always relevant or available for Drug Safety Update
documents.
Remove global wiring injection into Cucumber World

Replace a couple of naughty repository references with explicit calls to the
registry.
Remove AbstractAttachmentController subtypes

Instead create more registry objects and dynamically configure the controller
according to the document type coming in from the params.
Merge pull request #275 from alphagov/remove-dsu-validation

Remove validation of therapeutic_area from DSU
Merge pull request #277 from alphagov/remove-attachment-controller-subclasses

Remove AbstractAttachmentController subtypes
Merge pull request #278 from alphagov/remove-repository-sneaky-peaks

Remove global wiring injection into Cucumber World
DRY up exporters and formatters in Observers
Switch out delivery_api for email_alert_api

Start using the email_alert_api for sending emails from
specialist publisher in favour of the old delivery_api.