We have been using YouTube's 'hqdefault.jpg' image which works consistently to provide a 480x360 thumbnail. YouTube does provide larger thumbnails, but not consistently for every video. By using og:image, we will fetch the best resolution YouTube can provide for each video.
This commit also refactors lazy-yt to re-use the thumbnail already existing in the cooked content. This means we get lazy-loading for free, and avoid hotlinking images to YouTube (when download remote images is enabled on the site).
