fix and improve emailing class list aliases

[milescalabresi]

replace sha with hashlib for python3 to fix issue with emailing class list aliases

[willgearty]

Some additional ideas based on discussion:

1. We should verify sender email addresses in mailgate.py before sending along emails:

• https://pypi.org/project/dkimpy/
• https://pypi.org/project/checkdmarc/
• https://pypi.org/project/pyspf/

2. We should reject any emails from

• a sender email address that isn't associated with an account on [subdomain].learningu.org or learningu.org

[milescalabresi]

We discussed a better way to do it.

1. Catch sender's message and grab the data fields (To, From, Subject, Body, etc.)
2. If the sender's email is not associated with an account on the site:
   a. Do not forward the email
   b. Send the user a bounce message (rate-limit once per day/week/something using something similar to dbmail.MessageRequests to track)
3. If the sender's email is associated with an account on the site:
   a. If there is more than one such account, do some logic to identify the "right" account (options include: prefer admin > teacher > volunteer > student; choose earliest account created; send a bounce and instruct them to merge accounts), say [email protected]
   b. Use SendGrid to send an email to each recipient of the original message (To, Cc, Bcc) individually from [email protected]. We should be able to import and use the send_mail function

[willgearty]

With those changes, this should also fix #3760.

[willgearty]

I think you need to use instance.recipients to get the 'to' field

[milescalabresi]

Another good catch; added TODO