We now pass group in functions called while handling group update
events, instead of fetching the groups in those function as we
will need the old setting value in future commits.
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.
We already allow a user to join the group if they can add members
to that group in the API, but it was not allowed in the UI. This
commit fixes the UI.
