Add the notifier and the API endpoint for user shares

apps/federatedfilesharing/lib/Notifier.php

apps/files_sharing/appinfo/app.php

apps/files_sharing/appinfo/routes.php

apps/files_sharing/composer/composer/autoload_classmap.php

apps/files_sharing/composer/composer/autoload_static.php

apps/files_sharing/lib/AppInfo/Application.php

apps/files_sharing/lib/Controller/ShareAPIController.php

apps/files_sharing/lib/Notification/Listener.php

apps/files_sharing/lib/Notification/Notifier.php

lib/private/Share20/DefaultShareProvider.php