* 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
