Refactor scheduling tests

One test for each case: one for a new document; one for an updated document
Make the model type the worker expects clear

This worker only works with editions, so make that more explicit and simplify
the code.
Make the model type the presenter expects clear

This presenter only works with editions, so make that explicit
Add some coverage around scheduling translations

This adds a test to cover the scheduling of translated documents to make sure
that publish intents are published for each translation.
Remove redundant code

We now use the general purpose PublishingApiWorker for publishing all the things
and this Edition-specific one is no longer used.
Unscheduling editions with Publishing API

When a scheduled edition is unscheduled, we remove any publish intents that are
registered for it with Publishing API. If it is a first-edition, we also replace
the 'coming_soon' content item that will have been published when it was
scheduled with a 'gone' item.
Merge pull request #1921 from alphagov/unscheduling-publishing-2-0

Unscheduling publishing 2 0