Add intent presenter.

This will be used to send "intent to publish" messages to the content
store when an item is scheduled.
Publishing presenter for coming_soon format.
Bump gds-api-adapters to 17.2.0.
Wrappers for intent and coming_soon publishers.
Add publishing API worker for scheduled editions.

This is separate from the standard PublishingApiWorker because it has
two separate responsibilities: send an "intent to publish" to the
content-store intent endpoint, and additionally send a "coming soon"
format if there is currently nothing at that path.
Trigger PublishingApiScheduleWorker on scheduling.

Adds some integration tests for schedules.
Add coming_soon schema, plus validation tests.
Tidy up worker and tests.
Rename Intent to PublishIntent

This better matches the language elsewhere (e.g. content_store)
Add some basic documentation
Tweak test to ensure full coverage

We want to ensure that the publish intent is still sent, just that the "coming
soon" holiding page isn't.
Only expose and test external interface

the publish_coming_soon and publish_intent methods are for internal-use only.
Rename method for consistency and clarity
Remove redundant code

If the find fails, an exception will be raised
Merge pull request #1906 from alphagov/publish-schedules

Notify content-store when an edition is scheduled.