Modal invoice through btcpay.js (#381)

* Modal through btcpay.js

* Handling close action depending on whether is modal or not

* Tweaking button position

* Stripping trailing slashes if present when setting site root

BTCPayServer/Models/InvoicingModels/PaymentModel.cs

@@ -21,6 +21,7 @@ namespace BTCPayServer.Models.InvoicingModels
         public string CustomLogoLink { get; set; }
         public string DefaultLang { get; set; }
         public List<AvailableCrypto> AvailableCryptos { get; set; } = new List<AvailableCrypto>();
+        public bool IsModal { get; set; }
         public bool IsLightning { get; set; }
         public string CryptoCode { get; set; }
         public string ServerUrl { get; set; }