Send unpublished editions to the content store

Add a new format of "unpublishing" and use it to send editions when they are unpublished without an automatic redirect.
I think this makes the method is a little too clever as it's now doing two things: it is performing the action, and also saying whether the instance it was handed is publishable or not. Does it make more sense to raise an exception if an attempt is made to publish something that isn't publishable, especially now that we are able to publish both published and unpublished editions to the Publishing API? I would assume that the scope in data migration for this pull request would only return things that are publishable.