Preparatory work for permissions defined in terms of user groups, not roles

[timabbott]

This is a preparatory issue for a piece of work that will be eventually required to correctly implement Zulip's future user-group based permissions model. In zulip/zulip#21611, we add API support for clients to be able to receive in /register, and maintain via events, the set of user groups that each user is directly a member of, as well as any subgroups of those groups.

I'm guessing sometime this summer, the server will start adding permissions settings that are defined in terms of a user group that has access to do X, rather than a role; which would mean that client-side checks for whether a user has that permission would need to be able to do client-side queries of the form "Is the current user in group ID N".

I don't expect any such features or migrations of existing permissions settings to merge before Zulip 6.0 is released, and even if they do, they will likely only have roles as options for backwards-compatibility.