Running file: uri path to use

[vidartf]

If giving the server launch command a file path, it will try to open the browser at /notebooks/<file path>:

https://github.com/jupyter/jupyter_server/blob/4b8c9eebfcae24f4a09fcb796fbcfb1fa27a2950/jupyter_server/serverapp.py#L1580

The /notebooks part should probably be configurable (for user and/or extension apps).

If that is agreed, then comes the bike shed that is what to call the trait. A first suggestion could be file_to_run_url, but I'm happy for better suggestions.

[Zsailer]

You're right. This should be configurable.

[blink1073]

This was handled by #415