Please make notifications app configurable

[frankystone]

We are using and maintaining our own notification app, derived from notifications app v0.1.4 by James Tauber. We are doing this, because we are havily using the observed items functionality, which was removed from original notifications app some time ago.

Since commit https://github.com/arneb/django-messages/commit/8de4caa2a3b085d71758556890120fc598ec461a

updating django-messages will make use of our own app useless.

So a friendly ask if it would be possible to make the notification app configurable (instead of hardcoding it).

[arneb]

This is a valid point. We should indeed make it configurable.