Fixes#1428
Previously, we sent a single email with all recipients in the `bcc`
field, and no one in the `to` field. Mailgun does not support this.
This PR changes that behavior to send an email to each user using
Django's `send_messages` (plural) method.
Note: I wasn't able to confirm this in the Mailgun documentation, but I
assume this should not have any cost implications. Sending a "single"
email with 100 `bcc` recipients should cost the same as sending 100
emails each with a single `to` recipient.
Another note: I did not test performance of this with many recipients.
Probably all of our notification emails should be getting sent in a
Celery task... probably a future optimization.
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Co-authored-by: Greg Kaleka <gkaleka@energy-solution.com>
Part of #343. The news' notifications module was also changed to the user
preferences is honored when sending news emails.
Also exclude migrations from the black check.
