Send unpublished editions to the content store

Add a new format of "unpublishing" and use it to send editions when they are unpublished without an automatic redirect.
This is setting up exactly the same data as the test for the unpublishing presenter, which may be a bit confusing. It might make it clearer what the distinction is if this unpublishing has `redirect` explicitly set to true, and the one in the unpublishing presenter doesn't have an alternative url.