Asset form without sidebar

Created by: joeadams04

Lots of styling to be done, but submitting PR here so anyone that picks up next story to do with the form can start from here.

The sidebar can now be toggled on the Page component with the withSidebar prop. On pages that don't use the sidebar for nav we should use <NavigateBackButton /> component in the page's appbar.

This could probably be much smarter and implement more of this logic into one prop or higher level component, to avoid us manually adding the back button each time.