WooCommerce PDF Product Vouchers 3.12.3

* Fix - Email single header and subject headings were not used
* Fix - Convert special characters to HTML entities before PDF is generated to avoid potential character encoding issues

* Fix - Voucher templates not properly positioning existing voucher fields when edited
* Fix - New voucher templates not displaying the background image properly
* Fix - Upgrade to 3.1.2 did not finish for sites with more than 50 vouchers
* Fix - Customers unable to view vouchers in the account page introduced in 3.1.1

* Fix - Error in upgrade routine for version 3.1.1