Bug 1048353 - One time tasks need expiration date

Add management command and model method to close expired onetime attempts
While this actually looks pretty foolproof, I think it'd be better to use  a specified datetime here (like datetime(2014, 1, 1)) and then use `mock.patch` to patch `timezone.now` when you run the function to return that datetime. then you're 100% sure what's being returned as the datetime in the function.