Allow running play as a serverextension

[yuvipanda]

I want to run play as a serverextension, so I can provide it to users as an application in JupyterHub.

[rgbkrk]

Here's the sketch we drew last night:

A few things from our in-person conversations:

• Probably not a good idea to borrow much from the next.js app, only extract components we'd like to use amongst both (most are already in @nteract/core)
• Load direct contents paths /user/kyle/play/some/contents/backed/file.py
• Assume a python kernel (for now)
• Ok to start with a text-area, build out the monaco editor component after

(feel free to update the above)

[rgbkrk]

Some the unanswered questions:

• Should a new kernel be launched each time the user "runs" it?
• Do we save every time they run it? (When do we save?)