autocomplete: Follow new "Who can mention this group" setting for whether to include user groups #1776

In our initial implementation #1745 of user-group autocomplete #233, we just use is_system_group to decide which active user-groups should be included in autocomplete. This is correct for older servers, but modern servers include a can_mention_group field that we should use instead.

Prerequisites:

• Implement group-based permission checks #814

Implementation

Docs: search for can_mention_group: in https://zulip.com/api/register-queue