Send unpublished editions to the content store

Add a new format of "unpublishing" and use it to send editions when they are unpublished without an automatic redirect.
Why the `|| ""` here? I would have thought this would be undesirable as now an unpublishing that doesn't have an alternative url will have a blank string instead of `null` when loaded from the content store, meaning the frontend app now has to explicitly check for a blank string instead of a null value.