Opening settings and stream settings UI was not working for guests
if they could not access all users. This was because is_person_active
did not handle inaccessible users correctly, if they were not added in
the users data, when being called in get_group_members to render
group pills.
Removed `is_billing_admin` user property as it is no longer used since
billing permissions are now determined by `can_manage_billing_group`
realm setting.
