Cannot subscribe to Youtube Playlists

[mawise]

Youtube playlists have RSS feeds, but are not advertized.

Haven should observe: https://www.youtube.com/playlist?list=XYZZ

and map it to: https://www.youtube.com/feeds/videos.xml?playlist_id=XYZZ

[mawise]

Also add preview images for youtube videos: https://stackoverflow.com/questions/2068344/how-do-i-get-a-youtube-video-thumbnail-from-the-youtube-api

[mawise]

preview images are in #116