WooCommerce PDF Product Vouchers 3.12.3

* Tweak - Increase protection against unauthorized voucher downloads by using a nonce on the voucher download URL
* Tweak - Add system status tool to confirm if wp_remote_get works on the site
* Fix - PHP Error when duplicating a voucher
* Fix - PHP Warning when generating the voucher preview image

* Fix - PHP Errors when manually adding a new order

* Fix - Issue where customers could be shown the wrong purchased vouchers in the My Account area