make issue date field read only if the voucher is loaded on edit view

smash/web/forms/voucher_forms.py

@@ -80,6 +80,7 @@ class VoucherForm(ModelForm):
         self.fields['expiry_date'].required = False
         instance = getattr(self, 'instance', None)
         if instance and instance.pk:
+            self.fields['issue_date'].widget.attrs['readonly'] = True
             self.fields['voucher_type'].widget.attrs['readonly'] = True
             self.fields['hours'].widget.attrs['readonly'] = True
             self.fields['activity_type'].widget.attrs['readonly'] = True