Allow email notifications when creating invoices from Web UI (#2959)

Currently invoice email notifications are only sent when the invoice is created via the API. This commit adds an option to set an email address for notifications when an invoice is created from the Web UI.

BTCPayServer/Controllers/InvoiceController.UI.cs

@@ -868,6 +868,8 @@ namespace BTCPayServer.Controllers
                         Enabled = true
                     }),
                     DefaultPaymentMethod = model.DefaultPaymentMethod,
+                    NotificationEmail = model.NotificationEmail,
+                    ExtendedNotifications = model.NotificationEmail != null
                 }, store, HttpContext.Request.GetAbsoluteRoot(), cancellationToken: cancellationToken);
 
                 TempData[WellKnownTempData.SuccessMessage] = $"Invoice {result.Data.Id} just created!";