js: Use jQuery as a module.

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

static/js/portico/header.js

@@ -1,3 +1,5 @@
+import $ from "jquery";
+
 $(() => {
     $(".portico-header li.logout").on("click", () => {
         $("#logout_form").trigger("submit");
@@ -17,4 +19,3 @@ $(() => {
         }
     });
 });
-export {};