Ensure User model is unregistered before registering with Django admin

- PLAT-1631

common/djangoapps/student/admin.py

 """ Django admin pages for student app """
 from config_models.admin import ConfigurationModelAdmin
 from django import forms
+from django.contrib.admin.sites import NotRegistered
 from django.contrib.auth import get_user_model
 from django.contrib.auth.admin import UserAdmin as BaseUserAdmin
 from django.utils.translation import ugettext_lazy as _
@@ -207,4 +208,9 @@ admin.site.register(RegistrationCookieConfiguration, ConfigurationModelAdmin)
 
 
 # We must first un-register the User model since it may also be registered by the auth app.
+try:
+    admin.site.unregister(User)
+except NotRegistered:
+    pass
+
 admin.site.register(User, UserAdmin)