Handlers under nbclassic if notebook 7 is found

[echarles]

The goal of this PR is to allow both Notebook 7 as NbClassic to coexist in harmony and use the /tree, /notebook... in a wise way

If Notebook 7 if found being installed, NbClassic will register his handlers under a sub URL path /nbclassic. The default URL for nbclassic will be /nbclassic/tree, so if the user launches with jupyter nbclassic, the browser will be opened by default under /nbclassic/tree. And if the user launches with jupyter notebook, the browser will be opened by default under /tree and the user will see the Notebook 7 interface.

In case of presence of Notebook 7, the NbClassic (notebook < 7) extensions will continue working (tested with RISE and nbgrader). Some edge case may exist for extensions that would explicitely make a HTTP request to a hardcoded URL, but those extensions should not do that and should be updated to another mecansim.

[echarles]

This is ready for review. Please use "merge squash" if you approve it.

[RRosio]

This looks great! Thank you @echarles!

[jtpio]

Thanks both for working on this.

Any plans on making a new 0.4.4 release with this change, so it could be tested with the latest Notebook 7 pre-release?

[echarles]

Any plans on making a new 0.4.4 release with this change, so it could be tested with the latest Notebook 7 pre-release?

Yes, I have already added the need for nbclassic 0.4.4 to ship the latest commits. That will be useful also for notebook 6.5.0 final. Do you have a timeline in mind?

[jtpio]

Do you have a timeline in mind?

No concrete timeline, it's just so we can start testing having lab, notebook and nbclassic installed together again: jupyter/notebook#6547

[echarles]

No concrete timeline, it's just so we can start testing having lab, notebook and nbclassic installed together again: jupyter/notebook#6547

@jtpio nbclassic 0.4.4. is just released.

[jtpio]

Thanks, I just opened jupyter/notebook#6548 to see if that fixes it.