Promote visibility level helpers from Group to Namespace

In EE, we make use of `namespace#public?` in projects. When the project is in a
personal namespace, this breaks as the `public?` helper isn't present.

app/models/group.rb

@@ -2,7 +2,6 @@ require 'carrierwave/orm/activerecord'
 class Group < Namespace
   include Gitlab::ConfigHelper
-  include Gitlab::VisibilityLevel
   include AccessRequestable
   include Avatarable
   include Referable
@@ -103,10 +102,6 @@ class Group < Namespace
     full_name
   end
-  def visibility_level_field
-    :visibility_level
-  end
-
   def visibility_level_allowed_by_projects
     allowed_by_projects = self.projects.where('visibility_level > ?', self.visibility_level).none?

app/models/namespace.rb

@@ -5,6 +5,7 @@ class Namespace < ActiveRecord::Base
   include Sortable
   include Gitlab::ShellAdapter
   include Gitlab::CurrentSettings
+  include Gitlab::VisibilityLevel
   include Routable
   include AfterCommitQueue
@@ -105,6 +106,10 @@ class Namespace < ActiveRecord::Base
     end
   end
+  def visibility_level_field
+    :visibility_level
+  end
+
   def to_param
     full_path
   end

spec/models/namespace_spec.rb

@@ -63,6 +63,14 @@ describe Namespace, models: true do
     it { is_expected.to respond_to(:has_parent?) }
   end
+  describe 'inclusions' do
+    it { is_expected.to include_module(Gitlab::VisibilityLevel) }
+  end
+
+  describe '#visibility_level_field' do
+    it { expect(namespace.visibility_level_field).to eq(:visibility_level) }
+  end
+
   describe '#to_param' do
     it { expect(namespace.to_param).to eq(namespace.full_path) }
   end