Track membership of user groups

[gnprice]

In the initial implementation #662, we'll skip tracking the membership of the user groups, because that information isn't needed for our first application of the user-group data: #233.

This issue is for tracking the groups' membership, both direct and indirect. The prime application we'll need that data for is group-based permission checks, #814.

Copying from the #662 thread one wrinkle we'll need to handle when implementing this, to help make sure we remember:

For current servers since zulip/zulip#29498 (server-10, ZFL 303), correctly maintaining user groups will also involve acting on RealmUserUpdateEvent.isActive. See the API docs for that property.

Chat thread: https://chat.zulip.org/#narrow/stream/378-api-design/topic/Groups.20events.20for.20deactivated.20users/near/1958907

References

• Prerequisite: Store user groups #662
• A motivating use case: Implement group-based permission checks #814