Add autoretry_for to the mailer.send() task

[seanh]

Add autoretry_for=(Exception,) to the mailer.send() task and remove the manual retry-handling at the bottom of the task.

We want to retry sending an email whenever anything goes wrong, not only on smtplib.socket.error.

See autoretry_for in Celery's docs, and note that there's also dont_autoretry_for if we ever need it in future.