Fix a bug where the stream list would not get resorted if you unsubscribed and then subscribed from a stream

(imported from commit 9bde14f49d05062478fb5f356c24b29bedcdf3fc)

zephyr/static/js/stream_list.js

@@ -255,6 +255,8 @@ $(function () {
     $(document).on('subscription_remove_done.zephyr', function (event) {
         var stream_name = event.sub.name;
         exports.remove_narrow_filter(stream_name, 'stream');
+        // We need to make sure we resort if the removed sub gets added again
+        previous_sort_order = undefined;
     });
 });