Migrate UToronto off Classic Notebook to retrolab / notebook v7

[yuvipanda]

In https://2i2c.freshdesk.com/a/tickets/554 (#2380), we discovered that some of the jupyter classic notebook extensions in use at UToronto cause breakage due to the ongoing evolving situation with the jupyter_server migration. It was temporarily 'fixed' by reverting back to using NotebookApp, but that is not a long term solution.

Longer term, we should migrate off classic notebook to retrolab, which will smoothen the transition to notebook v7 (when it comes). data8 at Berkeley has been successfully using retrolab for more than a year for 1500+ students now, so I do think it is ready.

I think the primary work to be done is identifying what extensions are in use, and finding jupyterlab equivalents that do the trick. I think this should already exist for most things.

[pnasrat]

I'm wondering about the part of this that is identifying and finding equivalents that sounds like something that could be a generally usable standalone tool for upstream upgrades - cf 2to3 rather than something 2i2c specific.

[damianavila]

Writing jupyterlab/retrolab extensions is not straightforward by any means (the underlying tools changed dramatically) so porting nbextension (compatible with the legacy notebook) into the new lab-compatible extension format is, in most cases, not generalizable, and a case-by-case task. The good news is that a lot of extensions already migrated, the bad news is that several extensions still need porting (which is unlikely to happen for small or niche extensions).