Make favicon and logo images blocks in the template

IPython/html/templates/page.html

@@ -75,7 +75,7 b''
 
 <div id="header" class="navbar navbar-static-top">
   <div class="container">
-  <div id="ipython_notebook" class="nav navbar-brand pull-left"><a href="{{base_url}}tree" alt='dashboard'><img src='{{static_url("base/images/ipynblogo.png") }}' alt='IPython Notebook'/></a></div>
+  <div id="ipython_notebook" class="nav navbar-brand pull-left"><a href="{{base_url}}tree" alt='dashboard'>{% block logo %}<img src='{{static_url("base/images/ipynblogo.png") }}' alt='IPython Notebook'/>{% endblock %}</a></div>
 
   {% block login_widget %}