Turn admin menu sub-items into full-fledged objects

[elia]

Is your feature request related to a problem? Please describe.

The admin menu as defined in backend_configuration.rb correctly defines object for the first menu level but relies on views for submenu.

This leads to cumbersome development, either requiring too much effort or facilitating the introduction of bugs (e.g. see #3783).

Describe the solution you'd like

By using object for submenu items as well we could make the whole menu generated and let each menu item just define when it needs to be visible/active etc. and let the first level of menus just ask their collection of sub-items for such things.

So for example if we wanted to know if the settings menu should be visible we could just do settings_menu_item.items.any?(&:visible?), or to know if it should be considered "active" we would do settings_menu_item.items.any?(&:active?), etc.

Describe alternatives you've considered

The alternative would be to keep the current situation and manually fix everything, waiting for the next bug to be introduced 🐛

Additional context

#3783