Resolve inconsistent behaviour of the 'Site' menu item in the site editor navigation

[jameskoster]

This came up in #42807.

If you click 'Site' from the templates list it will collapse the nav panel and load the site homepage. However if you click 'Site' whilst editing a template then the nav panel remains open. This inconsistency makes navigating the site editor feel a bit inconsistent and fragile. Here's a video demonstration:

nav.mp4

Note how a user has to manually close the navigation to begin editing the site.

It might be good to try always closing the navigation panel when the 'Site' menu item is clicked.