Store user groups

[gnprice]

We should store the list of user groups in the realm, with their metadata and their memberships, and keep the data up to date.

This data originates as realm_user_groups in the initial snapshot at https://zulip.com/api/register-queue#response , and is updated by events.

As a preparatory refactor, we'll move PerAccountStore.users (which is a Map<int, User>, indexed by user ID) to live on a new type UserStore. This will closely follow the example of streams and StreamStore.

Then the user groups will be added as an additional data structure on UserStore.

This is a prerequisite for features involving user groups, in particular:

• autocomplete: Support user-group @-mentions #233

[gnprice]

We're likely about to add a client capability on the server related to user groups, with a name like allow_deactivated:
https://chat.zulip.org/#narrow/stream/378-api-design/topic/deactivated.20groups.20in.20.60GET.60.20endpoints/near/1940349

So when implementing this part of the API, we should start sending that client capability.

(In general, we should always be sending all client capabilities that have been defined so far. That way we get the latest version of the API, rather than an API that's been adapted for older clients.)

[gnprice]

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