confirm_dialog: Make Bootstrap fade class optional.

It would be better to load non-setting modals instantly.

This was previously reverted in
7bfa607d0a due to a rebase conflict.

static/js/settings_user_groups.js

@@ -368,6 +368,7 @@ export function set_up() {
             html_body,
             html_yes_button: $t_html({defaultMessage: "Confirm"}),
             on_click: delete_user_group,
+            fade: true,
         });
     });