Default statement transaction date

Fix #40

nummi/transaction/views.py

@@ -27,6 +27,9 @@ class TransactionCreateView(NummiCreateView):
             initial["statement"] = get_object_or_404(
                 self.request.user.statement_set, pk=self.kwargs["statement"]
             )
+            initial["date"] = (
+                initial["statement"].transaction_set.order_by("-date").first().date
+            )
 
         return initial