This commit adds code to disable checkbox in permissions panel
if unchecking the checkbox will set the setting to nobody group
and that setting can't be set to nobody group,
Fixes part of #33730.
Before this, we were using `is_user_in_setting_group` which does not do
the extra checks around a guest user's permissions (and in future, some
other checks).
We introduced `initialize_and_override_current_user` in stream_data test
to make it easier to set current_user.user_id.
We are using `can_add_subscribers_group` instead of
`invite_to_stream_policy` to check whether a user can subscribe other
users.
We've removed `invite_to_stream_policy` from the frontend wherever
applicable.
