text: Title: All users being allowed to register after "pre-authorized" list becomes empty
  Body: Assume that the "pre-authorized" parameter in config.yaml contains 10 email IDs. Now, if all 10 users (defined in the list) finish with their registration, their email IDs get deleted from "pre-authorized" and the **register_user** method starts allowing all users to register thereby defeating the purpose of this parameter. 
  
  Please correct me if I got this wrong. Thank you!