Use a notebook serverextension for culling

[yuvipanda]

We want to have a bit more intelligent culling, so let's make the single-user notebook pods do the culling themselves with a notebook extension. This lets us kill things when kernels have been idle for a while, rather than network request based killing.

[consideRatio]

Yes!

It would be great if the extension could reduce the following wasted use of resources situations.

• A user reschedules a singleuser pod by letting the computer come back from sleep, and since there was a browser window still open...
• A user might remain unculled without any actual activity.

I think JupyterLab might complicate things in general, something might work for normal notebooks, while not for a JupyterLab. Not investigated enough to tell.