Editions are sent to publishing API

Whitehall needs to notify the publishing API about published editions, so
that the notifications go on to the publishing message bus.

Editions are registered in the publishing API with a format of "placeholder".
Update Plek to use Plek.find insted of Plek.new.find or Plek.current
Adds PublishingApiTestHelpers

Changes tests to use publishing api test helpers
Add change_note and browse_page keys to attributes_for_publishing_api
Fixes failing unit test

* Force publishing now needs to take into account the PublishingApi
listener: test was updated to use the publishing API helpers

* Remove TimeCop since time is already frozen
Stub publishing API in features
Merge pull request #1734 from alphagov/sending-editions-to-publishing-api

Sending editions to publishing api