`None` in user display_name

Hi!

I haven't tracked down when/how this happens exactly, but I recently added a Moderator member and it got the string "None" as the user.display_name in the mailman3-db.

Afterwards, I could unsubscribe/resubscribe the member all I want, but the "None" display_name stays (because it's in the user.display_name).

After doing an explicit "DELETE /3.1/users/EMAIL" curl call, I finally got rid of the user and could reset the name when resubscribing. But it was trickier than I had hoped.

In short, there is one bug and one RFE:

• Sometimes the string "None" is stored for new users/members. Not sure when exactly.
• When you have a poorly named Member, it's not at all trivial to rename it. (Or is there some interface I have missed?)

Versions:

# mailman
mailman==3.2.2

# postorius
Django==2.0.13
django-mailman3==1.2.0
djangorestframework==3.9.2
HyperKitty==1.2.2
mailman-web==0.0.1
mailmanclient==3.2.2
networkx==2.3
postorius==1.2.4