Jupyter Preview not working locally. Config file should be jupyter_server_config not jupyter_notebook_config

[MarcSkovMadsen]

I'm on the current master branch of Panel with Jupyter lab 3.1.7, pyviz_comms 2.1.0 and bokeh 2.4.0.dev4

When running the jupyter lab preview it looks like

The server output looks like

$ jupyter lab
c:\repos\private\panel_docker\panel\.venv\lib\site-packages\jupyter_server\transutils.py:13: FutureWarning: The alias `_()` will 
be deprecated. Use `_i18n()` instead.
  warnings.warn(warn_msg, FutureWarning)
[I 2021-08-22 16:32:32.108 ServerApp] jupyter_server_mathjax | extension was successfully linked.
[I 2021-08-22 16:32:32.123 ServerApp] jupyterlab | extension was successfully linked.
[W 2021-08-22 16:32:32.143 ServerApp] The 'min_open_files_limit' trait of a ServerApp instance expected an int, not the NoneType 
None.

If I google The 'min_open_files_limit' trait of a ServerApp instance expected an int I find jupyterlab/jupyterlab#9752 (comment) i.e. we should migrate from jupyter_notebook_config.py to jupyter_server_config.py.

After some experimentation I find that the the jupyter_server_config.py should look like

{
  "ServerApp": {
    "jpserver_extensions": {
      "panel.io.jupyter_server_extension": true
    }
  }
}

and it works.

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