Allow NBClassic to work with NB7 and ServerApp.base_url

[mcrutch]

To test, upgrade to Notebook v7 and Lab v4, install this NBClassic, set ServerApp.base_url, run Jupyter, visit nbclassic

Also resolves an issue where New->New Notebook did not add the "/nbclassic/" in the url when Notebook v7 was installed

Personally not a fan of the var still being called base_url_prefix but I didn't want to change that unilaterally.

Closes #164

[echarles]

Thx a lot for this. Let's see what CI says.

Personally not a fan of the var still being called base_url_prefix

I am not either.

I didn't want to change that unilaterally.

Please do with a nicer name 👍

[mcrutch]

The concern with going down the path of changing base_url_prefix/document.baseUrlPrefix is that there were already places it was being used in nbclassic to detect running in "nbclassic" versus "Notebook v6" (see the about dialog) so could others have already keyed on that.

otherwise, nbclassic_path and 'document.nbclassicPathare ideas I had for a rename. if it is not running with NBClassic, the path is just '' otherwise it is '/nbclassic' Other option would be just carrying forward the name of the Python functionurl_prefix_notebook`

[echarles]

CI seems to be OK

I like nbclassic_path - If you go for it, it would be great to align the Python name also.

[mcrutch]

base_url_prefix, baseUrlPrefix and url_prefix_notebook() all updated to be nbclassic_path or nbclassicPath as appropriate

[echarles]

LGTM Thx @mcrutch