Umbrella flag should be per subscriber not per list

[Axel Beckert]

Currently a list can be either an umbrella list or not, but a list which has normal and other lists as subscriber is currently not possible or an administrative problem.

IMHO making the umbrella flag (as previously the moderated flag) per subscriber instead of per list, this would allow to make lists with mixed types of subscribers.

This would probably also make the following helpful automatisms possible or at least easier:

Mailman sould recognize mailing lists on the same server when they are subscribed to another list and set the umbrella flag for that subscriber automatically.

Mailman should automatically add the umbrella list's aliases as well as non-members which are allowed to post to the appropriate fields of the subscriber-lists on the same server. I suspect at least 90% of all subscriber-lists are configured that way.