implement per-list roster visibility options
This is for feature parity with Mailman 2.x.
Each list should have an option, changeable by the list owner, for three options for list roster visibility:
- the member roster is entirely public
- any list member can view the member roster
- only list moderators and above (list owners, domain owners, admins, superusers) can view the member roster
(In Mailman 2.x, a list moderator can approve pending subscriptions, msgs, and unsubscriptions. but cannot subscribe new members. If the roster is private only to list owners, then moderators can see the member roster.)
This needs to be implemented in Mailman core, be available via the core API, be implemented in mailman.client, and then be implemented in Postorius.