This MR adds the option to specify `priorityClassName` for pods in the chart and all included subcharts.

See the [Kubernetes documentation on PriorityClasses](https://kubernetes.io/docs/concepts/scheduling-eviction/pod-priority-preemption/#priorityclass) for detailed information.

Setting `global.priorityClassName` in the main `values.yaml` will result in all pods installed by this chart to be configured with the named `PriorityClass`, with the exception of GitLab Runner and Prometheus; these subcharts do not inherit `global.priorityClassName`.

A test has been added that sets `global.priorityClassName` to `system-cluster-critical`, and checks if all resulting `Deployments` and `Jobs` have `priorityClassName: system-cluster-critical` in their pod specs.

As a default installation does not include `StatefulSets`, `DaemonSets` or `CronJobs` these are not tested.

Changelog: added