Issue #3498922 by catch, m4olivei, plopesc: Use a placeholder for the navigation user block

core/modules/navigation/js/user-block.js

core/modules/navigation/navigation.libraries.yml

core/modules/navigation/src/Hook/NavigationHooks.php

...dules/navigation/src/Plugin/Block/NavigationUserBlock.php

...ion/tests/src/Functional/NavigationShortcutsBlockTest.php

...vigation/tests/src/Functional/NavigationUserBlockTest.php

...gation/tests/src/FunctionalJavascript/PerformanceTest.php