int_dict: Fix type errors for user_groups.

This fixes some harmless type errors from the
following commit:

    6ec5a1f306

The IntDict code automatically converts strings to
integers, so this was not a user-facing problem, but
we want to have our callers do the conversions
explicitly.

static/js/message_list_view.js

@@ -48,7 +48,7 @@ function get_user_group_id_for_mention_button(elem) {
     const user_group_id = $(elem).attr('data-user-group-id');
 
     if (user_group_id) {
-        return user_group_id;
+        return parseInt(user_group_id, 10);
     }
 
     return;