New functionality:

Users can now choose whether to use the current (\n) or the immediately previous (n-1) version of a notebook image when starting a notebook server, and we should update notebook server start procedures to reflect this option.

See internal doc here:

https://docs.google.com/document/d/104vGTJRb3l-NIcX7Dz6_3CHzVfqcU8XJ/edit