StatisticsAnnouncement returns recent change note

We want to display the change note of the most recent change that
required it as there may have been subsequent minor changes.
Wire in meta data for the previous display date

We want to include the display date prior to the last major change so
that users can be informed what the release date was prior to the
major change being made. Note that this code relies on the change note
only being set when absolutely necessary. The UI may need to be
updated such that entering a change note is only possible when the
change warrents it.
Cannot have a confirmed non-exact precision date
Rename method for consistency
Merge branch 'statistical-release-announcements' into announcable-statistics-frontend
update stats announcement provider to use last_change_note