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

Elwig/Documents/PaymentVariantSummary.cs

@@ -27,6 +27,7 @@ namespace Elwig.Documents {
             Data = data;
             CurrencySymbol = v.Season.Currency.Symbol ?? v.Season.Currency.Code;
             MemberNum = v.Credits.Count;
+            IsPreview = MemberNum == 0;
             DeliveryNum = v.DeliveryPartPayments.DistinctBy(p => p.DeliveryPart.Delivery).Count();
             DeliveryPartNum = v.DeliveryPartPayments.Count;
             ModifierStat = AppDbContext.GetModifierStats(v.Year, v.AvNr).GetAwaiter().GetResult();