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.