Notify content-store when an edition is scheduled.

On schedule (or force_schedule), send a notification to the content-store "publish intent" endpoint containing the schedule time. Additionally, if there is currently no content at the relevant path, send a "coming soon" format to the content endpoint.

This confused me initially because it looked like the worker that should be queuing the job to push things to the Publishing API was being stubbed, but then none of the code would work! Eventually I figured out that this is stubbing the _other_ worker that schedules the sidekiq job to perform the scheduled publishing on the current stack. I think this needs clarifying to avoid confusion. Maybe extract this to a private method called something like `stub_legacy_sidekiq_scheduling`.