Move some Utils functions to AppDbContext

Elwig/Windows/MemberAdminWindow.xaml.cs

@@ -94,7 +94,7 @@ namespace Elwig.Windows {
         }
 
         private async void InitInputs() {
-            MgNrInput.Text = (await Utils.NextMgNr(Context)).ToString();
+            MgNrInput.Text = (await Context.NextMgNr()).ToString();
             EntryDateInput.Text = DateTime.Now.ToString("dd.MM.yyyy");
             if (Context.Branches.Count() == 1)
                 BranchInput.SelectedItem = Context.Branches.First();