Documents: Add DeliveryConfirmation

Elwig/Documents/Document.cshtml.cs

@@ -100,6 +100,8 @@ namespace Elwig.Documents {
                 name = "DeliveryJournal";
             } else if (this is Letterhead) {
                 name = "Letterhead";
+            } else if (this is DeliveryConfirmation) {
+                name = "DeliveryConfirmation";
             } else {
                 throw new InvalidOperationException("Invalid document object");
             }