add custom footer license block (#159)

* add custom footer license block

* Apply suggestions from code review

Used same naming convention as the other blocks on the page (named after the enclosing div class, with underscores and 'extra' at the end)

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Co-authored-by: Sylvain Boissel <sylvain@boissel.dev>

dsfr/templates/dsfr/footer.html

@@ -117,6 +117,8 @@
              target="_blank"
              rel="noopener external"
              title="{{ etalab_license|capfirst }} - {{ new_window_label }}">{{ etalab_license }}</a>
+             {% block footer_bottom_extra %}
+             {% endblock footer_bottom_extra %}
         </p>
       </div>
     </div>