Add a task_done table and record every email sent in it

[seanh]

This should be similar to what LMS has, see lms/models/task_done.py.

task_done in h is not needed to avoid sending duplicate emails: the email-sending code does not need to check for the presence of a duplicate row in the task_done table before sending an email.

The task_done table will be useful as one way to monitor email-sending (enabling sent emails to be queried in Metabase) and will be used in future to implement rate-limits on emails by sender and by recipient.