Set Sharing

[martindale]

From #199:

Way to share entire playlists easily - generates a fun little bit.ly style URL along with a place to add a note for the person you are sharing it with. Can be member-based, but also sharable on other social media platforms and via e-mail.

It'd be great to make this a viral component for the site; building and sharing sets, which can be listened to immediately on target platforms (using embeds, appropriate APIs, etc.), could be a major contributor to our growth.