Replace django-celery-email

[jmsmkn]

The project is abandonware and the retry behaviour is causing problems. Browsing the code, we would only need to:

• Add our own email backend https://docs.djangoproject.com/en/5.0/topics/email/#defining-a-custom-email-backend
• Add our own email sending task, this can be an acks late task that runs on the micro short queue.

It would still use django-ses under the hood, but that is still well maintained.

[jmsmkn]

The most straightforward way to do this would be to use https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/ses/client/send_raw_email.html - then both dependencies could be replaced.

We could get over the celery message size limits by storing the email in the database, then sending a reference to that pk in the celery task.

[jmsmkn]

That was all straightforward. I'll run some tests on staging this afternoon.

[jmsmkn]

This appears to work well, we need to:

• [x] Remove all settings for the previous integrations
• [x] Remove the apps from poetry
• [x] Skip sending in development mode
• [x] Add tests
• [x] Add a periodic task to clean up older emails
• [x] Add a periodic task to send emails taking into account the rate limit
• [x] Bulk create the email objects when receiving multiple messages

[jmsmkn]

We could now also prevent queue flooding by having a periodic task that sends the email. This could also handle the SES rate limit.