Fix modal + form abuse (#34921)

See the comment. And due to the abuse, there is a regression: when the
modal is hidden, the form will be reset and it can't submit.

This PR fixes all problems: keep the modal with form open, and add
"loading" indicator.

web_src/js/features/comp/LabelEdit.ts

@@ -72,6 +72,7 @@ export function initCompLabelEdit(pageSelector: string) {
           return false;
         }
         submitFormFetchAction(form);
+        return false;
       },
     }).modal('show');
   };