App: Allow receipt to be shown in iframe (#6574)

Also detects if the checkout page is shown within an iframe and hides the back to store link in that case.

BTCPayServer/Controllers/UIInvoiceController.UI.cs

@@ -188,6 +188,7 @@ namespace BTCPayServer.Controllers
             return View(model);
         }
 
+        [XFrameOptions(null)]
         [HttpGet("i/{invoiceId}/receipt")]
         public async Task<IActionResult> InvoiceReceipt(string invoiceId, [FromQuery] bool print = false)
         {