add: setting of system notification

app/views/settings/show.json.jbuilder

@@ -56,4 +56,13 @@ json.setting do
   end
 
   json.common @common
+
+  if @top_system_notification.present?
+    json.system_notification do 
+      json.(@top_system_notification, :subject, :sub_subject, :content)
+      json.is_read @top_system_notification.read_member?(current_user.id)
+    end
+  else
+    json.system_notification nil
+  end
 end