[IMP] website: perf - prefetch menus when serving a page

If we are rendering a page, the menus will most likely be rendered as well.
Note that even in a 403 case when the page is found but is not visible, the
menus will also be shown on the 403 page.

Fetching the menus before accessing the requested page will prefetch that page
as well in one go if that page is in the menus, without any costs for the pages
not in the menus.

There is a tradeoff for 'non-layout' pages, which only occurs in advanced
technical cases:
1. Create a page with specific extension as name suffix (page.css) in which
   case the page will be bootstraped accordingly, without call to layout.
2. Remove the call to layout in HTML editor or backend
3. Remove the call to submenus template in HTML editor or backend
For those cases, the menus will be prefetched for no reason.

Note that homepage '/' is rendered through a controller, same logic is applied
there.

task-2211013

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