Simplify package setup by auto-adding everything needed in the app ready command

[joshuadavidthomas]

There are a bunch of steps one needs to take to use this package. Could be simplified by setting it all up in email_relay.apps.EmailRelayConfig.ready automatically, all someone would need to do is add email_relay to their INSTALLED_APPS list.

I've got a draft PR #23 that already does all this. Initially it's opt-in, so you would install the app and set DJANGO_EMAIL_RELAY["ENABLE_AUTO_SETUP"] = True. I could flip it to make it opt-out instead?

TBH, not totally sure on this one, may add too much magic and I'm not sure about the fragility of automatically adjusting settings outside of a project's settings.py. Though Carlton's https://github.com/carltongibson/django-template-partials package does this, so maybe I'm making a mountain out of a mole hill.

[joshuadavidthomas]

Pending the changes proposed in #154, this may not be needed. Using that approach of each application managing their own email queue in its local database, so the need for a database router goes away and the setup becomes much simpler. And since the email router would be marked for deprecation, we wouldn't necessarily need this change.

Closing for now.