portico: Add isolated_page context flag, to suppress site nav.

This flag allows rendering as a single isolated page, without the
navigation in header and footer that otherwise provides links to the
rest of the site.

The portico layout, including the styling of the "hero" area at top,
all remains the same.

We don't yet ever set this flag; that'll come next.

templates/zerver/landing_nav.html

@@ -1,3 +1,4 @@
+{% if not isolated_page %}
 <nav class="portico-header">
     <div class="content">
         <a class="brand logo" href="{{ root_domain_uri }}">
@@ -59,3 +60,4 @@
     </div>
     <svg height="32px" class="hamburger" style="enable-background:new 0 0 32 32;" version="1.1" viewBox="0 0 32 32" width="32px" xml:space="preserve" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"><path d="M4,10h24c1.104,0,2-0.896,2-2s-0.896-2-2-2H4C2.896,6,2,6.896,2,8S2.896,10,4,10z M28,14H4c-1.104,0-2,0.896-2,2  s0.896,2,2,2h24c1.104,0,2-0.896,2-2S29.104,14,28,14z M28,22H4c-1.104,0-2,0.896-2,2s0.896,2,2,2h24c1.104,0,2-0.896,2-2  S29.104,22,28,22z"/></svg>
 </nav>
+{% endif %}

templates/zerver/portico.html

@@ -10,14 +10,18 @@
 {% block content %}
 <div class="portico-container" data-platform="{{ platform }}">
     <div class="portico-wrap">
+        {% if not isolated_page %}
         {% include 'zerver/portico-header.html' %}
+        {% endif %}
         <div class="app portico-page {% block portico_class_name %}{% endblock %}">
             <div class="app-main portico-page-container{% block hello_page_container %}{% endblock %}">
                 {% block portico_content %}
                 {% endblock %}
             </div>
         </div>
     </div>
+    {% if not isolated_page %}
     {% include 'zerver/footer.html' %}
+    {% endif %}
 </div>
 {% endblock %}