Add migration to add authtokens for existing users
Add migration to add authtokens for existing users

* NB: Depending on how many users there are, this migration could be pretty RAM
  intensive...

WikiWatershed/model-my-watershed,WikiWatershed/model-my-watershed,WikiWatershed/model-my-watershed,WikiWatershed/model-my-watershed,WikiWatershed/model-my-watershed