Update PeerTube API for video-channels to exclude empty Channels

[mblomdahl]

For implementing OwnTube-tv/web-client#131 and OwnTube-tv/web-client#137, it would be beneficial to be able to exclude empty Channels 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-channels API endpoint to support passing the new filter?