Update all single field checkboxes

Includes adding some code to govukCheckboxesField
to add a single boolean-like option by default, if
there are no choices added.

app/templates/views/service-settings/email-reply-to/edit.html

@@ -29,9 +29,7 @@
       </p>
       {{ page_footer('Save') }}
     {% else %}
-      <div class="form-group">
-        {{ checkbox(form.is_default) }}
-      </div>
+      {{ form.is_default }}
       {{ page_footer(
         'Save',
         delete_link=url_for('.service_confirm_delete_email_reply_to', service_id=current_service.id, reply_to_email_id=reply_to_email_address_id),