User can create a group level cluster and install applications

⚠ Requires/includes the following MRs to be merged first :

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22307
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22438

What does this MR do?

Allow user to create group level cluster, install only Helm and Ingress apps and not much else. See also EE Merge request

This sits behind a group_clusters FF

What are the relevant issue numbers?

#34758 (closed)

What does this MR not include ?

• Show group clusters in project cluster list page
  • project maintainer might need to read cluster (but not read token !)
• Deploy to group clusters - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22308
• Modal to warn that it will override project cluster (well, it won't override in this MR)

Screenshots

(EE)

CE screenshots can be found below

Frontend

• Initialize pages JS for group clusters
• Install only Helm and Ingress

Backend and Frontend task

• Feature flag - hide Group > Operations
• Remove ability to set "Project Namespace"

Backend tasks

• Allow only Helm and Ingress installations
• Policy - cluster policy (group)
  • group maintainers != project maintainers
• extract *_path from views
• Controller
• views
  • Add new cluster button for group clusters
• Controller index spec for group
• service - CE can only create one cluster
• clusters/applications
  • push cluster/applications routes to top level
• Sidebar link
• Fix Clusters::Platforms::Kubernetes throwing Incomplete settings

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