Cancelling stats announcements

**Admin users can now cancel a statistics announcement**

* Cancelled announcements must have a reason for cancellation
* The cancellation reason can be edited after the fact
* Deleting an announcement is no longer possible
* Cancelled announcements are labelled as such in the front-end of the site

This delivers almost all of: https://www.agileplannerapp.com/boards/173808/cards/5477. The requirement to include cancelled announcements in the upcoming releases page for 1 month after their proposed release date will be delivered separately.

**Notes: Depends on https://github.com/alphagov/rummager/pull/281 being deployed.**


- [x] code review
- [x] product owner review