Send unpublished editions to the content store

Add a new format of "unpublishing" and use it to send editions when they are unpublished without an automatic redirect.
As long as there are no specialist sector tags in the unpublishing record sent to the content store, it won't appear in email alerts.

If we ever build an email alert of "everything published" (ie, no topic filter) this could be a problem - but maybe such an alert would want to know about unpublishings, too.  In any case, this should be fine for now.