Add ActivePageAny(ViewContext, IEnumerable<string>) so a dropdown
toggle gets the active class + aria-current="page" whenever any of
its children is the current route. Apply it to the Plateforme,
Administration, and account menu toggles.
Add PageHelpers.ActivePage extension and apply it to top-level nav
items and the Account/Register + Account/Signin items in _LoginPartial,
so the current route gets the active class and aria-current="page"
for accessibility.
