Use NamespacePolicy as base namespace policy class

- rename UserNamespacePolicy to NamespacePolicy
- subclass NamespacePolicy from BasePolicy
- subclass UserNamespacePolicy from NamespacePolicy

app/policies/namespace_policy.rb

@@ -8,3 +8,4 @@ class NamespacePolicy < BasePolicy
   # here.
   # See https://gitlab.com/groups/gitlab-org/-/epics/6689 for details
 end
+