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 release_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 does reflect that date range however.

Also, make a few of the tests a little less brittle.

Story: https://www.pivotaltracker.com/story/show/73205062
LGTM. can you confirm my understanding that all release dates are in the future, hence we're sorting in the ascending order so that one sees the next upcoming release at the top?