Start crond and add a crontab to run the Flarum Scheduler

[JamiePhonic]

In order for some Flarum Extensions to work correctly, the scheduler must be invoked by CRON.

This PR adds an S6 service to exexute crond -f as root, and adds a crontab with the required content to execute the scheduler. (Based on https://discuss.flarum.org/d/24118-setup-the-flarum-scheduler-using-cron)