Send unpublished editions to the content store

Add a new format of "unpublishing" and use it to send editions when they are unpublished without an automatic redirect.
The fact this is inheriting from the base Edition presenter makes me a little uncomfortable, as an unpublishing isn't really an edition. At the moment, this isn't causing an issue as the output generated matches our understanding of an unpublishing because the base class is being used for "placeholder" documents, but if the base Edition presenter ever stops being a "placeholder", then we'll have to untangle the unpublishing presenter because stuff like the "links" hash will be added. Something to be aware of.