Make "preferKernel" configurable in notebook-extension

[JasonWeill]

Problem

The attribute preferKernel is hard-coded to true in notebook-extension. If a user wants to override it to false, they need to fork the entire extension.

Proposed Solution

Add an option, either in the notebook-extension schema or in Page Config, to set preferKernel to false. Default it to true.

Additional context

Previously, this item referred to canStartKernel.

[JasonWeill]

@3coins @ellisonbg

[ellisonbg]

@jasongrout for kernel related logic.

[JasonWeill]

Renames to reflect preferKernel as the desired parameter to change.

[jasongrout]

Ah, adjusting preferKernel makes a lot more sense to me, rather than canStartKernel. So basically we're saying that it would then be easy to open notebooks without starting a kernel immediately, but you could still manually start a kernel if you wanted?

[JasonWeill]

So basically we're saying that it would then be easy to open notebooks without starting a kernel immediately, but you could still manually start a kernel if you wanted?

@jasongrout Yes, that's right. Some of my colleagues want this so much, they've forked JupyterLab to modify the function in #12260. Perhaps other users (admins) would benefit from this as well.

[jtpio]

open notebooks without starting a kernel immediately

Sounds like this would also help with this related issue in Notebook: jupyter/notebook#3170