Notifications for people and roles

https://www.pivotaltracker.com/story/show/59496412

Still some work to be done, but I'd like feedback on the implementation and test coverage.

The main goal is to allow signing up for alerts to feeds such as this https://www.gov.uk/government/ministers/prime-minister#announcements but as part of the process I'm cleaning up how alerts and triggers are built to reduce complexity.

- [x] Add trigger tags for people & roles
- [x] Tidy up filter dropdown helpers
- [x] Remove the alert creator and replace with a description of the alert you're signing up for
- [x] Properly describe non-document alerts, e.g. policies, people, roles
- [x] Pass through the local government flag when creating alerts
- [x] Tests for Whitehall::DocumentFilter::Options
- [x] Refactoring of Description#text
- [x] Does `relevant_for_local_government` apply for people & roles
- [x] Signup URL validation
- [x] Invoke `email_signup` validation in controller / view
- [x] Acceptance test to make sure that signup URLs are triggered when relevant editions are published
- [x] Remove the `@gov-uk-delivery-remembers-notifications` thing
- [x] Validate people & roles in `EmailSignup`
- [x] Extend acceptance test to cover documents
- [x] Extend acceptance test to cover topics
- [x] Extend acceptance test to cover topical events
- [x] Extend acceptance test to cover policies
- [x] Extend acceptance test to cover world locations
- [x] Extend acceptance test to cover organisations
- [x] Add support for existing topic email signup
- [x] Add support for existing organisation email signup
- [x] Add support for topical events
- [x] Add support for world locations
- [x] Fix legacy tests
- [x] Tidy up new signup interface

Possible `url_for` bug atom URLs having wonky params syntax, e.g. `publications.atom?&departments=...` might cause problems with subscriptions.

**Edit**: The URLs are parsed with Python's URL library which correctly deals with these URLs.