Create a link to the backend in the demo theme (octobercms#5100)

themes/demo/partials/site/header.htm

@@ -1,3 +1,9 @@
+==
+function onStart()
+{
+    $this['backendUrl'] = Backend::url('/');
+}
+==
 <!-- Nav -->
 <nav id="layout-nav" class="navbar navbar-inverse navbar-fixed-top navbar-autohide" role="navigation">
     <div class="container">
@@ -16,6 +22,7 @@
                 <li class="{% if this.page.id == 'home' %}active{% endif %}"><a href="{{ 'home'|page }}">Basic concepts</a></li>
                 <li class="{% if this.page.id == 'ajax' %}active{% endif %}"><a href="{{ 'ajax'|page }}">AJAX framework</a></li>
                 <li class="{% if this.page.id == 'plugins' %}active{% endif %}"><a href="{{ 'plugins'|page }}">Plugin components</a></li>
+                <li><a href="{{ backendUrl }}" target="backend">Sign in to the Backend</a></li>
             </ul>
         </div>
     </div>