add a note about disabling traditional sign-up in the new_app issue template

.gitlab/issue_templates/new_app.md

@@ -3,6 +3,8 @@
 ## Single sign-on
 
 * [ ] Integrate the new app into the signle sign-on system
+* [ ] Make sure the application does not allow registering through the
+  traditional sign-on system
 
 ## Documentation