Create a Mailman user when a user signs up
We should consider creating a Mailman User when a new user comes in. I think this would make a lot of sense looking at how people are actually deploying Mailman. It doesn't seem like people are actually integrating P & Hk with existing projects and I feel it would make some assumptions easier if we were to pre-create Mailman Core users per-signed up user in Postorius.
What I really want to get to is to allow people to sign up to mailing lists with their preferred addresses and be able to change that. It gets easier when the users are synced up and their preferred address in P is same as that in Core.