Add ability to detach external login services

data/model/legacy.py

@@ -1830,6 +1830,16 @@ def get_active_users():
 def get_active_user_count():
   return get_active_users().count()
 
+
+def detach_external_login(user, service_name):
+  try:
+    service = LoginService.get(name = service_name)
+  except LoginService.DoesNotExist:
+    return
+
+  FederatedLogin.delete().where(FederatedLogin.user == user,
+                                FederatedLogin.service == service).execute()
+
 def delete_user(user):
   user.delete_instance(recursive=True, delete_nullable=True)