Add dropdown to EDXAPP_REGISTRATION_EXTRA_FIELDS

For https://github.com/edx/edx-platform/pull/10413 , should it be accepted, though it will run without it.

playbooks/roles/edxapp/defaults/main.yml

@@ -413,6 +413,7 @@ EDXAPP_ANALYTICS_DASHBOARD_URL: "http://localhost:18110/courses"
 
 EDXAPP_REGISTRATION_EXTRA_FIELDS:
   level_of_education: "optional"
+  dropdown: "hidden"
   gender: "optional"
   year_of_birth: "optional"
   mailing_address: "optional"