EE: Top level clusters controller

What does this MR do?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22438 + fixing ee_compat

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/34758

Testing

• package-and-qa
• Test that cluster health for prometheus metrics are still appearing for a cluster

TODO

• Restore clusters#metrics.json but in a new controller

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?