Hi, please provide option for user preference to permanently hide header/toolbar.

From my perspective header is useless to me - it contains Jupyter logo and Notebook title, which I already have in my browser tab. Although it's height is reduced compared to early versions, why waste valuable vertical space?

I made JS start script to auto disable header/toolbar, but with latest version this HTML element is probably changed, and it is not the first time it is changed. So instead digging inside HTML DOM and writing JS, I thought to write to you.

Thanks