text: Title: All numeric registration code does not work
  Body: Registration codes that are all numeric don't work b/c we automatically cast it to an int. 
  
  This code needs a call to str so that we get a string always. 
  https://github.com/CTFd/CTFd/blob/514ab2c8bd3b0687615307c19fa9618b09e3998a/CTFd/auth.py#L214-L219