Added one crontab line for scheduled jobs, with the list in the code

Internal: the server's crontab needs one line, for bin/run-schedule.php, which every minute starts each job in ScheduledJobs that is due, as its own process under its own lock. Each run's start, finish and exit code are recorded, and Scheduled jobs in Admin shows whether the coordinator runs and how each job's last run went. A job whose entry is wrong is skipped and the others still run. It does nothing on a dev site. Mail's Send later is the first job; the other workers are still crontab lines of their own and move one at a time. See docs/SCHEDULED_JOBS.md.