Improve task to rebuild scheduled publishing queue

This currently cancels and rescheduled all editions due for publishing.
This won't catch any jobs scheduled for editions that are no longer due
to be published (ie orphaned sidekiq jobs).

Instead, this now clears all ScheduledPublisher jobs, and then
re-enqueues all scheduled editions.

This also includes a bit of refactoring to keep sidekiq knowledge inside the worker.

https://www.pivotaltracker.com/story/show/79398610