Send unpublished editions to the content store

Add a new format of "unpublishing" and use it to send editions when they are unpublished without an automatic redirect.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 find passing `nil` in here is a little cryptic. How about if it is set as the default in the method definition of `do_action` instead? e.g. 

```ruby
  def self.do_action(model_instance, update_type_override=nil)
    ...
  end
```