Backup duration alert: Get unit state and trigger times from systemd

Use a (bit) smarter Prometheus query to combine data from two different systemd units, the backup service and the backup timer: https://prometheus.io/docs/prometheus/latest/querying/operators/#vector-matching-keywords

Also in this patch: For the Backup dashboard - use data source names instead of UIDs.

Also in this patch: Small improvements regarding backup dashboard looks.

Obsoletes !352 (closed).