Add OARS to Kubrick

Summary: This also adds an OARS tag for app centers to know that Kubrick is safe for every user. https://hughsie.github.io/oars/ (for more description)

Reviewers: apol, #kde_games, ltoscano

Reviewed By: apol, ltoscano

Subscribers: ltoscano, kde-i18n-de, kde-games-devel

Tags: #kde_games

Differential Revision: https://phabricator.kde.org/D15641