Minor UI improvements (#2606)

* Added "Checkout" button to invoice detail page

* Added missing icon

All other pages have a "plus" icon on the primary button, except this page

* Button icons are always in front, not after like here was

BTCPayServer/Models/InvoicingModels/InvoiceDetailsModel.cs

@@ -116,5 +116,6 @@ namespace BTCPayServer.Models.InvoicingModels
         public List<PaymentEntity> Payments { get; set; }
         public bool Archived { get; set; }
         public bool CanRefund { get; set; }
+        public bool ShowCheckout { get; set; }
     }
 }