mysqlBackup service: let it work with default settings

* Grants enough privileges to the configured user so that it can run
  mysqldump.

* Adds a nixos test.

* Use systemd timers instead of a cronjob (by @fadenb).

* Creates a new user for backups by default, instead of using mysql
  user.

* Ensures that backup user has write permissions on backup location.

* Write backup to a temporary file before renaming so that a failed
  backup won't overwrite the previous backup, and so that the backup
  location will never contain a partial backup.

Breaking changes:

 * Renamed period to calendar to reflect the change in how to
   configure the backup time.

 * A failed backup will no longer result in cron sending an e-mail --
   users' monitoring systems must be updated.

Resolves #24728