Daily data push to front end

[hebron-george]

Problem

When the daily job runs to collect updates from TrackedPlaylists, we don't have a way of letting the front end know about the new songs without a full refresh of the front end.

The front end pings HYT on page load with POST /shuffle.

When a user uses the front end and leaves the app open in the browser, it won't update the playlist of available songs/videos without a full refresh.

We need a way for the back end to send a message of all the newly updated songs to any front end subscribers.

Related front-end issue: happy-software/shuffle_youtube_playlist#37