Update PeerTube API for video-playlists to exclude empty Playlists

[mblomdahl]

For implementing OwnTube-tv/web-client#138, it would be beneficial to be able to exclude empty playlists on the PeerTube server-side by passing ?hasPublicVideos=true, instead of filtering client-side. Can we please update the root-level GET /api/v1/video-playlists API endpoint to support passing the new filter?