Created three new celery tasks:
* save_sms (will replace send_sms)
* save_email (will replace send_email)
* save_letter (will replace persist_letter)
The difference between the new tasks and the tasks they are replacing is
that we no longer pass in the datetime as a parameter.
The code has been changed to use the new tasks, and the tests now run
against the new tasks too. The old tasks will need be removed in a separate
commit.