WooCommerce PDF Product Vouchers 3.12.4

* Tweak - Send voucher PDFs to purchasers in a separate email as attachments when generated
* Tweak - Redirect to the order received page when trying to download a voucher that isn't generated yet
* Fix - Compatibility issue with WooCommerce 2.6 when generating vouchers introduced in 3.2.0
* Fix - PHP warning when searching vouchers

* Fix - Fix a possible race condition when performing background processing health checks
* Fix - Account for possible false negatives when testing loopback connections in certain environments

* Feature - Offload PDF generation from checkout to a background job to vastly improve checkout performance, especially if multiple vouchers are purchased
* Tweak - Show an admin notice if voucher generation not supported
* Tweak - Save voucher activation date as postmeta
* Fix - Errors in v3.0 upgrade if voucher image attachments are not found or redemption amount is invalid
* Fix - Add line breaks in plain text email to voucher recipients for more readability
* Fix - Bypass tax calculation when taxes are completely disabled for the store
* Fix - Address an issue where voucher PDF generation failed on some PHP versions due to whitespaces between HTML tags
* Misc - Update DomPDF to 0.8.1