Enabling unpublishing for CIPs.

The method for looking up the Unpublishing object is different for CIPs, because
by default Unpublishings are searched by slug, and CIPs don't have unique slugs.
So we additionally search on organisation via the associated edition.
Test for CIP unpublishing.
Merge pull request #1580 from alphagov/unpublish_cips

Enable unpublishing for CIPs.