- GitLab Premium feature. Adds cluster_deployments license feature

- Finder for Cluster's environments. Returns a scope for the cluster's
environments. If no permission, returns a None scope

- Add a new serializer to represent environments for a cluster. These
can be for a group cluster so we cannot restrict to a single project
(like EnvironmentEntity currently does)

- Load environments for a cluster. We need it to return where the
environment's latest deployment is to a cluster. (and not the latest
deployment to that cluster).

- Add n+1 request spec.