Allow group displaynames in the database backend (#17221)

apps/provisioning_api/appinfo/routes.php

apps/provisioning_api/lib/Controller/GroupsController.php

core/Command/Group/Add.php

core/Migrations/Version18000Date20190920085628.php

lib/composer/composer/autoload_classmap.php

lib/composer/composer/autoload_static.php

lib/private/Group/Database.php

lib/private/Group/Group.php

lib/public/Group/Backend/IGetDisplayNameBackend.php

lib/public/Group/Backend/ISetDisplayNameBackend.php

lib/public/IGroup.php