Add a setting to enable the notebook to take up the full width

[jtpio]

Fixes https://github.com/jupyter/notebook/issues/6442 Fixes https://github.com/jupyter/notebook/issues/6407

• [x] Add a setting to enable the notebook to take the full width of the page. Defaults to false.
• [x] Add menu item
• [x] Add to the command palette
• [x] Add UI test?

notebook-full-width-setting.webm

[github-actions[bot]]

:point_left: Launch a Binder on branch jtpio/notebook/full-width-notebook