Refactor API::Settings for EE

This refactors API::Settings so that EE can more easily extend its
behaviour, without having to modify the CE source code directly.

lib/api/settings.rb

@@ -9,6 +9,11 @@ module API
         @current_setting ||=
           (ApplicationSetting.current_without_cache || ApplicationSetting.create_from_defaults)
       end
+
+      def filter_attributes_using_license(attrs)
+        # This method will be redefined in EE.
+        attrs
+      end
     end
 
     desc 'Get the current application settings' do
@@ -156,6 +161,8 @@ module API
         attrs[:password_authentication_enabled_for_web] = attrs.delete(:password_authentication_enabled)
       end
 
+      attrs = filter_attributes_using_license(attrs)
+
       if ApplicationSettings::UpdateService.new(current_settings, current_user, attrs).execute
         present current_settings, with: Entities::ApplicationSetting
       else