Support YouTube shortlink urls

[goldenapples]

YouTube has started using URLs of the form

https://youtu.be/aDGBYbiIzKg?list=PLxd0bZ1RXEzuPCekFpssGpsyUdNSNFLNX

as the share link for playlists. We should update the URL parsing in our shortcode to accept this format as the specified URL, and convert it to the more standard format:

https://www.youtube.com/watch?list=PLxd0bZ1RXEzuPCekFpssGpsyUdNSNFLNX&v=aDGBYbiIzKg