Checkout v2 (#4157)

* Opt-in for new checkout

* Update wording

* Create invoice view update

* Remove jQuery from checkout testing code

* Checkout v2 basics

* WIP

* WIP 2

* Updates and fixes

* Updates

* Design updates

* More design updates

* Cheating and JS fixes

* Use checkout form id whenever invoices get created

* Improve email form handling

* Cleanups

* Payment method exclusion cases for Lightning and LNURL

TODO: Cases and implementation need to be discussed

* Introduce CheckoutType in API and replace UseNewCheckout in backend

Co-authored-by: nicolas.dorier <nicolas.dorier@gmail.com>

BTCPayServer.Client/Models/CreateAppRequest.cs

@@ -36,6 +36,8 @@ namespace BTCPayServer.Client.Models
         public string RedirectUrl { get; set; } = null;
         public bool? RedirectAutomatically { get; set; } = null;
         public bool? RequiresRefundEmail { get; set; } = null;
+        public string CheckoutFormId { get; set; } = null;
         public string EmbeddedCSS { get; set; } = null;
+        public CheckoutType? CheckoutType { get; set; } = null;
     }
 }