Fix redirects on existing Unpublishings.

Tidy up old Unpublishings that no longer validate according to current rules.

* Fix alternative_urls that point to whitehall-admin instead of gov.uk;
* Change instances of deprecated unpublishing_reason_ids 2 and 3 to currently valid 1 or 4, and change automatic redirect as necessary.
Is there a reason this isn't simply correcting the host to `www.gov.uk` for these attachments?