Implementation of the custom email backend to handle SES events

[albertisfu]

This is a custom email backend to handle sending an email with some extra functions before the email is sent.

• Verifies if the recipient's email address is not banned before sending or storing the message.
• Stores a message in DB, generates a unique message_id.
• Verifies if the recipient's email address is under a backoff event waiting period.
• Verifies if the recipient's email address is small_email_only flagged or if attachments exceed MAX_ATTACHMENT_SIZE, if so send the small email version.
• Compose messages according to available content types

It's necessary to set the following settings:

• BASE_BACKEND: The base email backend to use to send emails, in our case: django_ses.SESBackend, for testing, is used: django.core.mail.backends.locmem.EmailBackend
• MAX_ATTACHMENT_SIZE: The maximum file size in bytes that an attachment can have to be sent.

The backend supports storing only one email address for the recipient, bcc, and cc fields.
The verifications that the backend performs (email banned and backoff events) are not supported for messages with one recipient.

I saw that in general CL messages are sent to only one recipient, however, I found this command:
cl.alerts.management.commands.monitor_pacer

That seems to send emails to multiple recipients in the same message, using this email backend messages will be sent, however since we only store one recipient, verifications for email banned and backoff events are not supported.

One approach to solve this problem could be to change this command to send one message for each recipient, like this one:
cl.users.management.commands.cl_welcome_new_users

[mlissner]

I only did a partial review for now, but a couple high level comments and a few detailed once as well:

1. I think the backend applies beyond just users, so I'd put it in cl.lib.email_backends.py instead. (Note that I made it plural too, in case someday we have another backend for some reason.)

2. Should we have an admin page for the Email table?

3. Wow, tests!

Thanks for this. Looking good.

[albertisfu]

Thank you so much for your comments!
I've followed your recommendations and applied some improvements.
Now we are using the last release of django-ses 3.0.1!

[mlissner]

Lots of little stuff. I still don't love some of the deep nesting and long functions you have here. I think we may just decide it's fine, but one opportunity I did see was to simplify the store_message and compose_message functions.

Anyhow, let me know what you think.

[albertisfu]

Thank you for your comments!

I think these changes are ready, I left some comments above, let me know if there is something more to change.

There's a problem with our black version to parse the new parenthesized context managers #1994 (comment)

[mlissner]

Tests are still going, but this isn't a merge into main, so I'm going to merge. Thanks for the good work here.