js: Convert static/js/notifications.js to ES6 module.

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

static/js/unread_ops.js

@@ -2,6 +2,7 @@
 
 const channel = require("./channel");
 const message_viewport = require("./message_viewport");
+const notifications = require("./notifications");
 const overlays = require("./overlays");
 const reload = require("./reload");
 const unread_ui = require("./unread_ui");