Refactor XFrames Attribute & simplify pos settings page (#576)

* Enable better error when invoice cannot be created on crowdfund

Closes #572

* Allow all public apps in iframe

* cleanup pos page dev info

BTCPayServer/Models/AppViewModels/UpdatePointOfSaleViewModel.cs

@@ -41,5 +41,7 @@ namespace BTCPayServer.Models.AppViewModels
         [MaxLength(500)]
         [Display(Name = "Custom bootstrap CSS file")]
         public string CustomCSSLink { get; set; }
+
+        public string Id { get; set; }
     }
 }