Make notebook 6.5.x point to nbclassic static assets

[ericsnekbytes]

This makes notebook use the static assets of nbclassic, so both use the same physical files for future frontend development. Main changes (still being refined) are:

• Change the DEFAULT_STATIC_FILES_PATH in __init__ to point to nbclassic/static folder
• Delete the static/ folder from notebook
• Remove static file build/related logic from setup system (nbclassic will do this itself, we don't need the logic here anymore)
• Remove JS tests from CI (those tests are covered in nbclassic)

Note that this PR involves removing static frontend JS/CSS files from notebook and obtaining those instead from nbclassic, it does not remove the handlers and other logic, those are still in place.

[echarles]

I have installed locally this branch and launching jupyter notebook I receive

  File ".../jupyter-notebook/notebook/__init__.py", line 29, in <module>
    from notebook.nbextensions import install_nbextension
ModuleNotFoundError: No module named 'notebook.nbextensions'

[echarles]

Epic PR @ericsnekbytes Tested locally and looks good. Checked the diff. CI failures were also shown before on 6.4.x branch. Will merge. Thx again!

[echarles]

@ericsnekbytes I have reviewed and approved. Can you undraft so I can merge? Thx,

[ericsnekbytes]

@ericsnekbytes I have reviewed and approved. Can you undraft so I can merge? Thx,

Ready!