Stats announcements index ordered by public_timestamp

Stats announcements (https://whitehall-admin.preview.alphagov.co.uk/government/statistics/announcements) didn't have any explicit ordering before. This PR orders them by public_timestamp instead.

The visible announcement date is not the same as the announcement's timestamp, as in many cases the announcement has a displayed date range of up to two months instead of a discrete date.  The timestamp should reflect that date range however.

There aren't any test with this as it really is just a hard-coded addition to a search query.

Story: https://www.pivotaltracker.com/story/show/73205062