js: Convert static/js/billing/upgrade.js to ES6 module.

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

.eslintrc.json

@@ -274,7 +274,6 @@
                 "unread": false,
                 "unread_ops": false,
                 "unread_ui": false,
-                "upgrade": false,
                 "upload": false,
                 "upload_widget": false,
                 "user_events": false,

static/js/billing/upgrade.js

@@ -1,6 +1,4 @@
-"use strict";
-
-exports.initialize = () => {
+export const initialize = () => {
     helpers.set_tab("upgrade");
 
     const add_card_handler = StripeCheckout.configure({
@@ -82,8 +80,6 @@ exports.initialize = () => {
     helpers.update_charged_amount(prices, $("input[type=radio][name=schedule]:checked").val());
 };
 
-window.upgrade = exports;
-
 $(() => {
-    exports.initialize();
+    initialize();
 });

static/js/global.d.ts

@@ -140,7 +140,6 @@ declare let ui_util: any;
 declare let unread: any;
 declare let unread_ops: any;
 declare let unread_ui: any;
-declare let upgrade: any;
 declare let upload: any;
 declare let upload_widget: any;
 declare let user_events: any;