stream_data: Use has_metadata_access for can_view_subscribers.

We are using `has_metadata_access` one by one in different functions in
stream_data where possible. This commit does not represent the
exhaustive use of `has_metadata_access`.

We have kept around the `can_view_subscribers` function even though it
is the same as `has_metadata_access` right now. Since we've already done
the work of using `can_view_subscribers` at appropriate places, it would
be good to keep that work around in case that function's underlying
implementation changes in the future.

web/src/stream_data.ts

@@ -596,8 +596,7 @@ export function can_edit_description(sub: StreamSubscription): boolean {
 }
 
 export function can_view_subscribers(sub: StreamSubscription): boolean {
-    // Guest users can't access subscribers of any(public or private) non-subscribed streams.
-    return current_user.is_admin || sub.subscribed || (!current_user.is_guest && !sub.invite_only);
+    return has_metadata_access(sub);
 }
 
 export function can_subscribe_others(sub: StreamSubscription): boolean {

web/tests/stream_data.test.cjs

@@ -1232,6 +1232,7 @@ test("can_unsubscribe_others", ({override}) => {
         stream_id: 1,
         can_remove_subscribers_group: admins_group.id,
         can_administer_channel_group: nobody_group.id,
+        can_add_subscribers_group: nobody_group.id,
     };
     stream_data.add_sub(sub);