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.
* The `rummager:index:statistics_announcements` rake task will need to be run shortly after deploy to seed upcoming announcements with the new `statistics_announcement_state` attribute in rummager.

- [ ] code review
- [ ] product owner review
This Looks like it's not going to handle stats announcements which are in the index and shouldn't be. Does something need to be added to here: https://github.com/alphagov/whitehall/blob/master/lib/whitehall.rb#L125 to ensure that these stats announcements are reindexed when the index is reset (following rake tasks reset:government => rummager:index:government)?