Use NamespacePolicy as base namespace policy class

- subclass NamespacePolicy from BasePolicy
- subclass UserNamespacePolicy from NamespacePolicy
- subclass ProjectNamespacePolicy from NamespacePolicy
- update ProjectNamespace specs to disallow everything

app/policies/namespace_policy.rb

 # frozen_string_literal: true
 
-class NamespacePolicy < ::Namespaces::UserNamespacePolicy
+class NamespacePolicy < BasePolicy
   # NamespacePolicy has been traditionally for user namespaces.
   # So these policies have been moved into Namespaces::UserNamespacePolicy.
   # Once the user namespace conversion is complete, we can look at
   # either removing this file or locating common namespace policy items
   # here.
+  # See https://gitlab.com/groups/gitlab-org/-/epics/6689 for details
 end

app/policies/namespaces/project_namespace_policy.rb

 # frozen_string_literal: true
 
 module Namespaces
-  class ProjectNamespacePolicy < BasePolicy
+  class ProjectNamespacePolicy < NamespacePolicy
     # For now users are not granted any permissions on project namespace
     # as it's completely hidden to them. When we start using project
     # namespaces in queries, we will have to extend this policy.

app/policies/namespaces/user_namespace_policy.rb

 # frozen_string_literal: true
 
 module Namespaces
-  class UserNamespacePolicy < BasePolicy
+  class UserNamespacePolicy < ::NamespacePolicy
     rule { anonymous }.prevent_all
 
     condition(:personal_project, scope: :subject) { @subject.kind == 'user' }

spec/policies/namespaces/project_namespace_policy_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-RSpec.describe NamespacePolicy do
+RSpec.describe Namespaces::ProjectNamespacePolicy do
   let_it_be(:parent) { create(:namespace) }
   let_it_be(:project) { create(:project, namespace: parent) }
   let_it_be(:namespace) { project.project_namespace }
@@ -37,7 +37,7 @@ RSpec.describe NamespacePolicy do
     let_it_be(:current_user) { create(:admin) }
 
     context 'when admin mode is enabled', :enable_admin_mode do
-      it { is_expected.to be_allowed(*permissions) }
+      it { is_expected.to be_disallowed(*permissions) }
     end
 
     context 'when admin mode is disabled' do