Cluster environments BE/FE integration

What does this MR do?

This is the integration MR for https://gitlab.com/gitlab-org/gitlab-ee/issues/13392

CE Port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32541

• Backend: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15096
• Frontend: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15037

TODO:

• provide cluster_environments_url as a data-attribute
• Backend/Frontend integration
• Remove feature flags
  • Frontend
• Documentation for GitLab Premium feature
  • This feature is called "Cluster Environments"
  • It is available for GitLab Premium customers
  • This is available for group-level clusters for now
  • Instance-level clusters is planned in https://gitlab.com/gitlab-org/gitlab-ce/issues/63985
  • We provide a view of which projects and environments deployed to the cluster
  • Maybe mention this is not all environments, only environments deployed to this cluster ?
  • Access : only group maintainers and owners, ie who can view clusters
  • Also include pod count for each environment. Criteria for pod to be included is same as deploy board