traits in notebook that other extensions might depend on

[Zsailer]

After a conversation with @blink1073 this morning,

I looked at traits in NotebookApp that other extensions might depend on. If we turn the NotebookApp into server extension, such traits/configurations won't be seen by other server extensions. This would break those extensions.

Here is the full list:

extra_nbextensions_path
enable_mathjax
max_body_size
max_buffer_size
webapp_settings
server_extensions
nbserver_extensions
notebook_dir
mathjax_url
get_secure_cookie_kwargs
base_project_url
mathjax_config
ignore_minified_js

The only trait that causes trouble in JupyterLab is notebook_dir. Jupyter server renames this trait to root_dir.

We may want to add notebook_dir to jupyter_server with a deprecation warning.

[krinsman]

Copy-pasted from my comment on the PR to fix this:

Will deprecating and then eventually removing notebook_dir make this incompatible with JupyterHub, and thus prevent jupyter_server from being used as a backend for jupyterhub-singleuser/SingleUserNotebookApp?

JupyterHub uses it in a pretty essential way, since that's what c.Spawner.notebook_dir hooks into:
https://github.com/jupyterhub/jupyterhub/blob/master/jupyterhub/spawner.py#L467
https://github.com/jupyterhub/jupyterhub/blob/master/jupyterhub/spawner.py#L930

[Zsailer]

This is handled by the new shims in NBClassic. Apps that switch dependency from notebook to jupyter_server should use these shims.