Big refactorying for supporting multi currencies

BTCPayServer/Data/HistoricalAddressInvoiceData.cs

@@ -17,6 +17,17 @@ namespace BTCPayServer.Data
             get; set;
         }
 
+
+        [Obsolete("Use GetCryptoCode instead")]
+        public string CryptoCode { get; set; }
+
+#pragma warning disable CS0618
+        public string GetCryptoCode()
+        {
+            return string.IsNullOrEmpty(CryptoCode) ? "BTC" : CryptoCode;
+        }
+#pragma warning restore CS0618
+
         public DateTimeOffset Assigned
         {
             get; set;