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/shared.py

@@ -0,0 +1,18 @@
+import features
+
+from data import model
+
+def can_create_user(email_address):
+  """ Returns true if a user with the specified e-mail address can be created. """
+  if not features.USER_CREATION:
+    return False
+
+  if features.INVITE_ONLY_USER_CREATION:
+    if not email_address:
+      return False
+
+    # Check to see that there is an invite for the e-mail address.
+    return bool(model.team.lookup_team_invites_by_email(email_address))
+
+  # Otherwise the user can be created (assuming it doesn't already exist, of course)
+  return True