Navigation bar: allow excluding extras (#617)

Port of carpentries/instructor-training@dc1b52a

Co-authored-by: maneesha sane <[email protected]>

Co-authored-by: maneesha sane <[email protected]>

Author: maxim-belkin

_includes/navbar.html

@@ -80,7 +80,9 @@
             {% else %}
               {% assign extra = lesson_extra %}
             {% endif %}
-            <li><a href="{{ relative_root_path }}{{ extra.url }}">{{ extra.title }}</a></li>
+            {% unless extra.exclude %}
+              <li><a href="{{ relative_root_path }}{{ extra.url }}">{{ extra.title }}</a></li>
+            {% endunless %}
             {% endfor %}
           </ul>
         </li>