Support rendering private notebooks in the job detail page

[maurodoglio]

As the title says, it would be great to have private notebooks rendered in the job details page. At the moment that's not supported because the notebook files are retrieved directly from s3.

[jezdez]

This is somewhat related to #739 which would solve this in a better way. The current "fetch notebook file from S3, pipe it through ATMO and then display it using Javascript" mode was really a shortcut and it shows .