BTCPay Server Extensions (#1925)

* BTCPay Server Extensions

![demo](https://i.imgur.com/2S00aL2.gif)

* cleanup

* fix

* Polish UI a bit,detect when docker deployment

BTCPayServer/Services/Notifications/Blobs/InvoiceEventNotification.cs

@@ -1,6 +1,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using BTCPayServer.Configuration;
+using BTCPayServer.Contracts;
 using BTCPayServer.Controllers;
 using BTCPayServer.Events;
 using BTCPayServer.Models.NotificationViewModels;