Include a new policy in Clusterables
(projects and groups), which checks if another cluster
can be added
clusterable_has_cluster? and multiple_clusters_available
private methods will be overriden in EE
Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/34758
- Move show path for cluster to ClusterPresenter
- Create ClusterablePresenter to encapsulate logic. Consolidates
scattered methods from BaseController and ClustersHelper into an object.