Use UserNamespacePolicy for user namespaces

Currently user namespaces are handled by Namespace model. If we introduce proper UserNamespace model for user namespaces, it would be good to extract user-specific rules from NamespacePolicy to Namespaces::UserNamespacePolicy and keep only rules shared by all types of namespaces in NamespacePolicy.

So eventually we can use something like:

{Project|Group|User}NamespacePolicy < NamespacePolicy