Complete rehaul of cron job setup.
Key improvements:
- (Hopefully) Fixes cron jobs not triggering sometimes
- Jobs delayed for a random amount of minutes to make sure we don't spam letsencrypt's servers.
- Use /etc/cron.weekly/ instead of /etc/cron.d/
- Cron job log files are now in /var/log/letsencrypt-renew-certs
- Logrotate makes sure that the log files get rotated every month