Notifications for people and roles

**NO NOT MERGE - FEEDBACK ONLY**

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.

- [ ] Add trigger tags for people & roles
- [ ] Tidy up filter dropdown helpers
- [ ] Remove the alert creator and replace with a description of the alert you're signing up for
- [ ] Properly describe non-document alerts, e.g. policies, people, roles
- [ ] Remove local government alerting entirely from the govuk delivery trigger code (handled elsewhere)
- [ ] More tests?