add nbviewer button to jupyterhub

notebook.patch

diff --git a/notebook/templates/page.html b/notebook/templates/page.html
index 9e68da1b2..a5539711b 100644
--- a/notebook/templates/page.html
+++ b/notebook/templates/page.html
@@ -133,7 +133,13 @@ dir="ltr">
   {% endblock %}
   
   {% block header_buttons %}
-  
+ 
+  {% block nbviewer_button %}
+  <span id="nbviewer_widget">
+      <button onclick="createLink()" id="logout" class="btn btn-sm navbar-btn">Nbviewer</button>
+  </span>
+  {% endblock %}
+
   {% block login_widget %}
 
     <span id="login_widget">
@@ -189,6 +195,13 @@ dir="ltr">
       }
     }
   }
+ function createLink() {
+    var p1 = /\/notebooks/
+    var p2 = /\/user\/|tree/g
+    var pathname = window.location.pathname.replace(p1, '').replace(p2, '')
+    url = 'https://your.host.here.com:8000/services/nbviewer/localfile/' + pathname
+    window.open(url, '_blank');
+  } 
   _remove_token_from_url();
 </script>
 </body>