add django-dkim

[jsnshrmn]

Description

sign django emails directly in our app; This should be ready to deploy since I've added appropriate dns records and docker secrets for prod.

Rationale

Our unsigned emails are getting dropped

Phabricator Ticket

https://phabricator.wikimedia.org/T347512

How Has This Been Tested?

I manually updated stating to set me as the contact address and to use this backend instead of the console so that I could easily trigger an email. It now passes validation checks.

Screenshots of your changes (if appropriate):

Types of changes

What types of changes does your code introduce? Add an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Minor change (fix a typo, add a translation tag, add section to README, etc.)