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.