Move entity type and ID from base path and into routing parameters

Overview

In ✨ [PP-1] Connect "new page" with API to create new pages Postponed I wanted to switch the editor to a different page. To do so I need to refresh the page by moving to a new URL and re-mounting the editor. That is because the entity type and ID are part of the base path for the editor. It is entirely contextual to one entity.

Proposed resolution

Have /xb be the base path and the editor has the entity type and ID as part of its routing. This will break a lot of things, probably, like moving to /editor or /preview, etc. But maybe the useNavigation hook can simplify this by always using the entity type and ID and a different hook for changing the main entity being edited.

User interface changes