All Products for WooCommerce Subscriptions 6.0.4

* Update - Added support for adding carts to existing subscriptions via the checkout page.

* Fix - Fixed an issue that prevented carts with one-time purchase products from being added to existing subscriptions.
* Fix - Fixed an issue that prevented plan discounts from applying when adding a cart to an existing subscription.
* Fix - Fixed an issue that prevented Bundle scheme prices from calculating correctly when bundled item discounts applied to Regular product prices

* Tweak - Declared incompatibility with the new High-Performance Order Storage.
* Tweak - Improved security.