Deploy boards for group-level Kubernetes clusters

Problem to solve

Deployment board functionality is not currently available in group-level k8s clusters.

Target audience

developers, operators

Further details

See https://gitlab.com/gitlab-org/gitlab-ce/issues/55103#further-details and https://gitlab.com/gitlab-org/gitlab-ce/issues/55103#note_124860370

Proposal

Provide the same deployment board functionality currently available for project-level clusters.

What does success look like, and how can we measure that?

Links / references

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28613