Avoid building a big set of allowed_users if allow_all is configured

Co-authored-by: Min RK <[email protected]>

oauthenticator/oauth2.py

@@ -610,7 +610,7 @@ def add_user(self, user):
         """
         if not self.validate_username(user.name):
             raise ValueError("Invalid username: %s" % user.name)
-        if self.allow_existing_users:
+        if not self.allow_all and self.allow_existing_users:
             self.allowed_users.add(user.name)
 
     def login_url(self, base_url):