- MenuManager::getMenuItems(): undefined constant as default value
  (self::ALL_ELEMENTS, since the copy of prodigious/sonata-menu in step 1b),
  evaluated by the debug tools in dev: 500 error on the home page, hence the
  Symfony error page JS error and the toolbar 404 loop.
  ConstantExpressionsTest evaluates all default values and constants of src/.
- Admin stylesheet: default asset package (Encore manifest), Sonata Admin
  4.3x uses its own package for extra_stylesheets: 404 with a prod build.
- Admin layout: jQuery UI datepicker initialisation removed (jQuery UI is no
  longer loaded since Sonata 4, $.datepicker was undefined; the fields are
  native HTML5 date fields).