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/stream_create.js

@@ -372,7 +372,7 @@ export function set_up_handlers() {
             confirm_dialog.launch({
                 html_heading: $t_html({defaultMessage: "Large number of subscribers"}),
                 html_body,
-                on_click: () => {
+                on_click() {
                     create_stream();
                 },
             });