Further improvements for automatic repository checks

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As discussed with @pcarranza

• RepositoryCheck::BatchWorker should abort its run if more than 10 projects failed their check
• BatchWorker should schedule a single delayed retry for each failed project (safe because we limit the number of failures per batch)
• create RepositoryCheck::ScheduleWorker and put that in sidekiq-cron instead of BatchWorker. To avoid issues where sidekiq-cron restarts / terminates BatchWorker.