eslint: Enable object-shorthand avoidExplicitReturnArrows option.

This is equivalent in the absence of ‘this’ (which ESLint knows to
check for).

Signed-off-by: Anders Kaseorg <anders@zulip.com>

static/js/dialog_widget.js

@@ -153,7 +153,7 @@ export function launch(conf) {
 
     overlays.open_modal("dialog_widget_modal", {
         autoremove: true,
-        on_show: () => {
+        on_show() {
             if (conf.focus_submit_on_open) {
                 $submit_button.trigger("focus");
             }