Document: Add IsPreview to indicate that a document may only be viewed by internal staff

Elwig/Documents/CreditNote.cs

@@ -39,6 +39,7 @@ namespace Elwig.Documents {
             Data = data;
             Payment = p;
             Credit = p.Credit;
+            IsPreview = Payment == null || Credit == null;
             var season = p.Variant.Season;
             if (considerCustomModifiers) {
                 CustomPayment = ctx.CustomPayments.Find(p.Year, p.MgNr);