Use Sidekiq for scheduled publishing

Replaces the cron-based scheduled publishing code with sidekiq scheduled jobs. We'll need to plan the deploy of this for a time when there aren't any editions due for publishing and run the `publishing:scheduled: queue_missing_jobs` rake task to queue jobs for any existing scheduled editions.

Story: https://www.agileplannerapp.com/boards/173808/cards/3416
Yes, that would probably be a good idea. Fixed.