User registration newsletter subscription checkbox

Future releases of this module might implement newsletter subscription feature.
We will try to make it as user friendly as we can, to provide basic feature out of the box, and also as developer friendly as possible for overriding. Thanks.

Very cool @renrhaf nice to see progress here!
I created ✨ Brevo newsletter (un)subscribe API Active as a first step to postpone the UI / form features on it.