Our app is now "base path"-proof :-) - fixes #1

* added a "define_javascript_app_config" hook and a "get_app_javascript_config" Twig function, so that we give some data to our app JavaScript part - including the app base path, for RequireJS proper scripts loading
* removed the "display_app_perfs_info" Twig function, replaced with a simple include of "utils/debug/app-perfs-info.twig"