When a post moves from //published on a blog// to //draft//, we should federate the change.
Right now if we send out a `Delete` activity, we get the desired effect. But platforms like Mastodon don't do anything if we later send another `Create` activity with the same post ID (for example, by re-publishing the post). Need to see if we can do anything better, or if these platforms need to change.
Another option: versioning the post ID? Would add a lot of complexity, but might fix this.