Rake task to requeue all scheduled publishing jobs.

At the moment, after a database restore, the list of scheduled editions
does not match the list of sidekiq jobs in redis.  There is an existing
task to add jobs for any editions that have mising jobs.  This works for
some cases, however it fails to account for editions that are no longer
scheduled (orphaned sidekiq jobs), or editions that have have their
scheduled time changed.

To address this, we therefore delete all sidekiq jobs for the
ScheduledPublishingWorker and then re-queue all editions that are
scheduled for publication.
Merge pull request #1747 from alphagov/requeue_all_scheduled_rake_task

Rake task to requeue all scheduled publishing jobs.