Group-level cluster web terminal access

Problem to solve

Group-level Kubernetes clusters don't currently support terminal functionality (which is currently supported in project-level clusters)

Target audience

developer, operator

Further details

See https://gitlab.com/gitlab-org/gitlab-ce/issues/55103

Proposal

Provide the same terminal functionality we currently provide in project-level clusters.

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

Links / references