Navigation block: Delay creating entities until changes are saved

[scruffian]

What problem does this address?

Following on from #39883 it would be great if changes to navigation menus weren't converted to entities until the "Save" button in the Site Editor is clicked. This will avoid creating new navigation menus when the user is just making changes for testing, which should not be saved.

[getdave]

@scruffian Can we close this one?

[scruffian]

Yeah I think we've got a better path forward.