title: Jupyter Creating Notebook failed: FORBIDDEN
body: <p>I have just upgraded Jupyter to the version 4.3.1
  While I can open previously created ipynb files, I cannot create new ones.</p>
  
  <p>When I try to create a new notebook file, I get a pop up windows saying:<br>
  Creating Notebook Failed<br>
  An error occurred while creating a new notebook<br>
  Forbidden</p>
  
  <p>In the terminal I notice this output:</p>
  
  <pre><code>[W 12:53:23.375 NotebookApp] 403 POST /api/contents (::1): '_xsrf' argument missing from POST
  [W 12:53:23.383 NotebookApp] 403 POST /api/contents (::1) 8.92ms referer=http://localhost:8888/tree?token=e7fbbb58516dc1359fcc26a1079093166a1f713ee5b94ccd
  </code></pre>
  
  <p>I use Jupyter with Python 3.5.2 and IPython 5.1.0</p>