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/StoreBaseData.cs

@@ -31,6 +31,10 @@ namespace BTCPayServer.Client.Models
         public bool AnyoneCanCreateInvoice { get; set; }
         public string DefaultCurrency { get; set; }
         public bool RequiresRefundEmail { get; set; }
+
+        public string CheckoutFormId { get; set; }
+        [JsonConverter(typeof(StringEnumConverter))]
+        public CheckoutType CheckoutType { get; set; }
         public bool LightningAmountInSatoshi { get; set; }
         public bool LightningPrivateRouteHints { get; set; }
         public bool OnChainWithLnInvoiceFallback { get; set; }
@@ -66,6 +70,12 @@ namespace BTCPayServer.Client.Models
         public IDictionary<string, JToken> AdditionalData { get; set; }
     }
 
+    public enum CheckoutType
+    {
+        V1,
+        V2
+    }
+
     public enum NetworkFeeMode
     {
         MultiplePaymentsOnly,