Add a service to find out if a user knows another user

apps/provisioning_api/lib/Controller/UsersController.php

apps/provisioning_api/lib/Listener/UserDeletedListener.php

...provisioning_api/tests/Controller/UsersControllerTest.php

lib/composer/composer/autoload_classmap.php

lib/composer/composer/autoload_static.php

lib/private/KnownUser/KnownUserMapper.php

lib/private/KnownUser/KnownUserService.php