Only provide token link if token is set and user logged in

notebook/templates/page.html

@@ -118,7 +118,10 @@ dir="ltr">
 
 <div id="header">
   <div id="header-container" class="container">
-  <div id="ipython_notebook" class="nav navbar-brand pull-left"><a href="{{default_url}}?token={{token}}" title='{% trans %}dashboard{% endtrans %}'>{% block logo %}<img src='{{static_url("base/images/logo.png") }}' alt='Jupyter Notebook'/>{% endblock %}</a></div>
+  <div id="ipython_notebook" class="nav navbar-brand pull-left"><a href="{{default_url}}
+    {%- if logged_in and token -%}?token={{token}}{%- endif -%}" title='{% trans %}dashboard{% endtrans %}'>
+      {% block logo %}<img src='{{static_url("base/images/logo.png") }}' alt='Jupyter Notebook'/>{% endblock %}
+  </a></div>
 
   {% block headercontainer %}
   {% endblock %}