Fix the default notebook windowing mode

[jtpio]

#7312 added a plugin to default to the defer windowing mode, because the full windowing mode still has CSS issues:

• CSS issue when the Notebook windowing mode is set to full #7231
• Issue #7231 Update base.css #7301

We should look into removing it. Or find a way to use defer but for Notebook 7 only, without setting the new value in the settings.

In #7312 (comment) @krassowski suggests:

It looks like you would need to override the notebookConfig in factory: https://github.com/jupyterlab/jupyterlab/blob/6152e3a5f94d728ccd2727d6d8413f12ddafb669/packages/notebook-extension/src/index.ts#L1867-L1898

An alternative would be to use setting transforms to change the default value temporarily.

[jtpio]

Seeing this behavior when trying in a fresh environment on Binder, from https://gist.github.com/jtpio/befb8ebf2b630b4748e2538f79877022:

notebook-default-defer.webm