Use new Sanitizable concern in NamespaceSetting

- Includes and uses new concern
- Adds specs to capture behaviour

app/models/namespace_setting.rb

@@ -2,6 +2,7 @@
 
 class NamespaceSetting < ApplicationRecord
   include CascadingNamespaceSettingAttribute
+  include Sanitizable
 
   cascading_attr :delayed_project_removal
 
@@ -25,6 +26,8 @@ class NamespaceSetting < ApplicationRecord
 
   self.primary_key = :namespace_id
 
+  sanitizes! :default_branch_name
+
   def prevent_sharing_groups_outside_hierarchy
     return super if namespace.root?
 
@@ -34,11 +37,7 @@ class NamespaceSetting < ApplicationRecord
   private
 
   def normalize_default_branch_name
-    self.default_branch_name = if default_branch_name.blank?
-                                 nil
-                               else
-                                 Sanitize.fragment(self.default_branch_name)
-                               end
+    self.default_branch_name = default_branch_name.presence
   end
 
   def default_branch_name_content

spec/models/namespace_setting_spec.rb

@@ -3,6 +3,8 @@
 require 'spec_helper'
 
 RSpec.describe NamespaceSetting, type: :model do
+  it_behaves_like 'sanitizable', :namespace_settings, %i[default_branch_name]
+
   # Relationships
   #
   describe "Associations" do
@@ -41,14 +43,6 @@ RSpec.describe NamespaceSetting, type: :model do
 
         it_behaves_like "doesn't return an error"
       end
-
-      context "when it contains javascript tags" do
-        it "gets sanitized properly" do
-          namespace_settings.update!(default_branch_name: "hello<script>alert(1)</script>")
-
-          expect(namespace_settings.default_branch_name).to eq('hello')
-        end
-      end
     end
 
     describe '#allow_mfa_for_group' do