astakos: Refactor user creation
Provide auth.make_user() as the single way to create a new user. This function is responsible to set all automatically generated fields, such as username, uuid, and token. Clean up AstakosUser.save(), that used to update these fields, and remove AstakosUser.__init__(). Remove trigger that renewed token on every AstakosUser update. In order to set a user's email, use AstakosUser.set_email(); this takes care to update the username, too. Provide function create_user() in user creation forms, which calls auth.make_user() with the form-provided data. Use the wrapper auth.make_local_user() in management command `user-add'. Use the same infrastructure to extend a django superuser to an AstakosUser (in management command `fix-superusers').