Send unpublished editions to the content store

Add a new format of "unpublishing" and use it to send editions when they are unpublished without an automatic redirect.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.e. raise a custom exception with a helpful message:

```ruby
  unless publishable?(model_instance)
    raise UnpublishableInstance.new("#{model_instance.class} with ID #{model_instance.id} is not publishable") 
  end
```