Plugin changes authentication in http requests to local files

[pgorczak]

I've run into this via bokeh/jupyter_bokeh#42 and vega/altair#927.

When ipympl is installed, http requests to local files (GET /files/...) made by the vega plugin stop working. They get forwarded to /login with a 302 status code. Installing ipympl seems to cause the requests to be sent without any cookies. In my case requests without ipympl present had cookie data _xsrf, csrftoken, username-localhost-8888, username-localhost-8889.