Add feature flag to allow users to be created only if invited to join a team

Allows for open user creation, but only if extended an invitation by someone who already has access

data/users/federated.py

@@ -4,6 +4,7 @@ import features
 from collections import namedtuple
 
 from data import model
+from data.users.shared import can_create_user
 from util.validation import generate_valid_usernames
 
 logger = logging.getLogger(__name__)
@@ -99,7 +100,7 @@ class FederatedUsers(object):
     db_user = model.user.verify_federated_login(self._federated_service, username)
     if not db_user:
       # We must create the user in our db. Check to see if this is allowed.
-      if not features.USER_CREATION:
+      if not can_create_user(email):
         return (None, DISABLED_MESSAGE)
 
       valid_username = None