Add initial cell identifiers awareness

[krassowski]

To keep the conversation from #553 going, in particular the step 1:

step 1, ignore cell id and make a release with unchanged behavior that is aware of cell ids, as if it were any other opaque metadata field. Since cell IDs are hidden from the user, I think showing them in diffs should also be done at the changed-metadata level, maybe hidden by default.

I am opening a draft PR that might help with conceptualizing what is needed to move forward. The number of failing tests is down from 34 to 32 (fixed test_pretty_print_code_cell and test_git_diff_driver_add_helper_filter I think) but the bulk of merge tests is still failing. They are now failing for a different reason (I temporarily set "/cells/*/id": "remove" while awaiting feedback to highlight the file where the changes are likely needed).

I really wish we could get nbdime out of beta to also allow jupyterlab-git to be properly released and for the lab ecosystem users to migrate to 3.0 and reduce the maintenance burden on the older versions of other Jupyter software too.

This PR might help to close #553.

[vidartf]

Thanks for picking up the slack here. I have some changes locally that complements this (test changes only). I'll do a PR to your branch with them.

[krassowski]

All tests that were failing due to introduction of cell ids are now passing. There are still failing tests, but unrelated:

• test_git_difftool[jupyter_server]: {'mathjaxUrl': ''} != {'mathjaxUrl': '/static/components/MathJax/MathJax.js'}
• test_offline_mathjax: tornado.httpclient.HTTPClientError: HTTP 404: Not Found

[krassowski]

Comparing jupyter_server/search?q=mathjax_url and notebook/search?q=mathjax_url it looks that:

• the handler was kept in the jupyter_server, but
• the actual mathjax javascript was removed from jupyter_server and is only present in notebook: notebook/static/base/js/mathjaxutils.js
• the mathjax config was also removed from jupyter_server and is only present in notebook: notebook/notebookapp.py#L1344-L1365

[vidartf]

Thanks! 🥇