This adds a function that sends provided email to all administrators
of a realm, but in a single email. As a result, send_email now takes
arguments to_user_ids and to_emails instead of to_user_id and
to_email.
We adjust other APIs to match, but note that send_future_email does
not yet support the multiple recipients model for good reasons.
Tweaked by tabbott to modify `manage.py deliver_email` to handle
backwards-compatibily for any ScheduledEmail objects already in the
database.
Fixes#10896.
92dc3637df