Add model/interface for cancelling announcements

Inlining styles straight from fofr's mockups for now. Will rationalise
the CSS in a later commit.
Remove delete from stats announcements

Users should not be able to delete an announcement once it's been
created as we do not want to lose the permenant record of an
announcement. Instead, announcements that are no longer valid should be
cancelled.
Restrict actions on cancelled announcements

You should not be able to cancel or change the release date on a
cancelled statistics announcement.
Allow editing of the cancellation reason
Strike-through cancelled release date
Update stats announcements with cancellation info
Include cancellation details in frontent model