Optional delay of grandchildren link display

The current default is that when a grandparent is selected, the links to its
children *and all its grandchildren* are displayed in the navigation menu.
Setting `grandchildren_branch: true` in `_config.yml` delays the display of links
to grandchildren until their parent is selected.

_includes/nav.html

@@ -20,6 +20,7 @@
                     {%- endif -%}
                     <a href="{{ child.url | absolute_url }}" class="navigation-list-link{% if page.url == child.url %} active{% endif %}">{{ child.title }}</a>
                     {%- if child.has_children -%}
+                    {%- if page.url == child.url or page.parent == child.title or site.grandchildren_branch != true -%}
                         {%- assign grand_children_list = site.html_pages | where: "parent", child.title | sort:"nav_order" -%}
                         <ul class="navigation-list-child-list">
                         {%- for grand_child in grand_children_list -%}
@@ -29,6 +30,7 @@
                         {%- endfor -%}
                       </ul>
                     {%- endif -%}
+                    {%- endif -%}
                   </li>
                 {%- endfor -%}
               </ul>