Implements what jupyter/notebook#3116 makes possible

[maartenbreddels]

Enable nbextension at installation, only makes sense to merge when jupyter/notebook#3116 is merged.

[vidartf]

I see jupyterlab (https://github.com/jupyterlab/jupyterlab/pull/3546/files) uses etc/jupyter/jupyter_notebook_config.d. Are there any guidelines for what path to use?

[bollwyvl]

So in that case, jupyterlab only needs to add itself as a serverextension, so it only adds a file to jupyter_notebook_config.json's counterpart, i.e. jupyter_notebook_config.d.

If you needed to add an nbextension, and add a serverextension, you might have:

jupyter-config/
  jupyter_notebook_config.d/
    my-extension.json
  nbconfig/
    notebook.d/
      my-extension.json

and the data_files would be something like:

('etc/jupyter/jupyter_notebook_config.d', 
 'jupyter-config/jupyter_notebook_config.d', 
 'my-extension.json'),
('etc/jupyter/nbconfig/notebook.d', 
 'jupyter-config/nbconfig/notebook.d', 
 'my-extension.json'),

How you lay it out on disk is up to you, but something like this structure will likely make it more maintainable as soon as you have more than one of them running around.

As to the actual name of the file... no hard and fast rule, but you know, unique.