Store template ID and original file name in URL

At the moment you can’t press refresh on the check page if there’s
errors. This is because the session gets cleared when there’s errors.
This is a bad user experience.

The data that this page is relying on (from the session) is:
- template ID
- original file name

Neither of these things need to be in the session because:
- they are not secret
- the user can modify them already (by choosing a different template or
  renaming their file locally)

So this commit additionally stores them in the URL.

app/templates/views/notifications/check.html

@@ -53,7 +53,7 @@
         {% if template.template_type != 'letter' or not request.args.from_test %}
         <button type="submit" class="button">Send 1 {{ message_count_label(1, template.template_type, suffix='') }}</button>
         {% else %}
-          <a href="{{ url_for('main.check_messages_preview', service_id=current_service.id, template_type=template.template_type, upload_id=upload_id, filetype='pdf') }}" download class="button">Download as a printable PDF</a>
+          <a href="{{ url_for('main.check_messages_preview', service_id=current_service.id, template_id=template.id, upload_id=upload_id, filetype='pdf') }}" download class="button">Download as a printable PDF</a>
         {% endif %}
       {% endif %}
       <a href="{{ back_link }}" class="page-footer-back-link">Back</a>