WooCommerce Waitlist 2.4.16

* Fix: Waitlist counts for variations not showing correctly

* Fix: Full pass adding validations where required to correct edge cases where product objects were assumed to be properly loaded, leading to errors

* Fix: Email templates hooks not passed in the correct parameters. "woocommerce_email_header" and "woocommerce_email_footer" now have the email class object passed in as required
* Added: Notification added to inform administrator that waitlist templates are out of date. This will be added as required on theme switch and plugin update