Helper for user preference

This MR provides a React-like helper for user preference. Plus a rework of the main sidebar to use it for the collapsed state.

This also fixes the getCache function in order to be able to store false/null/undefined, which was previously read back as {}.

This easily can be used to replace a React useState.

const [state, setState] = React.useState(null)

const [state, setState] = useUserPreference(['foo', 'bar', '2000'], null)