Add drug safety updates

This PR adds the files necessary for MHRA editors to publish Drug Safety Updates. The commits roughly go as follows:

- Add schema
- Add Model, Validator and Builder
- DRY up the routes so we only need to add a new string for a whole new set of routes
- add DSU to the Wiring
- add formatters
- add permission checks for MHRA
- add controllers
- add observers and services
- add DSU views and routes
- testing DSU creating, editing, publishing and withdrawing

[Ticket](https://trello.com/c/4drpVQbv/275-dsu-finder-add-support-to-specialist-publisher-for-publishing-drug-safety-updates-2). Relies on https://github.com/alphagov/govuk_content_models/pull/217 being merged and Panopticon being bumped.