Documents: Replace Razor templates with iText

Elwig/Documents/Letterhead.cs

@@ -2,8 +2,9 @@ using Elwig.Models.Entities;
 
 namespace Elwig.Documents {
     public class Letterhead : BusinessDocument {
-        public Letterhead(Member m) : base($"Briefkopf {m.FullName}", m, true) {
-            Aside = "";
+        public Letterhead(Member m) :
+            base($"Briefkopf {m.FullName}", m, true) {
+            Aside = null;
         }
     }
 }