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/Models/BitpayCreateInvoiceRequest.cs

@@ -81,6 +81,9 @@ namespace BTCPayServer.Models
         public bool? RedirectAutomatically { get; set; }
         [JsonProperty(PropertyName = "requiresRefundEmail", DefaultValueHandling = DefaultValueHandling.Ignore)]
         public bool? RequiresRefundEmail { get; set; }
+        
+        [JsonProperty(PropertyName = "checkoutFormId", DefaultValueHandling = DefaultValueHandling.Ignore)]
+        public string CheckoutFormId { get; set; }
 
         //Bitpay compatibility: create invoice in btcpay uses this instead of supportedTransactionCurrencies
         [JsonProperty(PropertyName = "paymentCurrencies", DefaultValueHandling = DefaultValueHandling.Ignore)]