Consider changing module name to `django_email_relay`

[joshuadavidthomas]

Per Adam Johnson's recommendation on the Django forum:

I propose changing this page to recommend a module name of django_polls and making the admonition warn against not using the django_ prefix. The current approach causes name clashes which prevents using packages together.

https://forum.djangoproject.com/t/change-reusable-apps-naming-recommendation/25233

We would need to break it out in to two separate releases -- one to add the new module and have the old one alias back to the new with a deprecation warning, then another to remove the old email_relay module.