backups: Implement scheduled backups
This MR proposes simplified approach for scheduling backups:
- Each repository is assigned to exactly one schedule. Schedule can be disabled (default).
- There are only three periods for backup: daily, weekly and monthly. Each can be disabled.
- Backups are cleaned up automatically based on return values.
- Borgmattic a backup scheduler for Borg was considered and rejected. Provided value is not worth the integration effort.
- Notifications are shown for errors and to encourage a new user to setup a backup schedule.