automate generation of playlists

[ldang]

By default, all videos uploaded to SCALE should belong to a particular playlist. For example, SCaLE 21x videos should belong to the SCaLE 21x playlist.

However, the way that people attend SCaLE, they often pick particular tracks that they are interested in. So it would be good if we could automatically put videos into additional playlist by track.

For example, all workshops could go into a SCaLE 21x Workshops track. All Embedded talks could go into a SCaLE 21x Embedded track.

NOTE: We need to be careful about storing a canonical mapping between our playlist ID and track names, as you can make many playlists of the same name.

An added requirement is to sort the playlist talks in a deterministic order, hopefully by room, and by room schedule. It would help preserve the feel of the track, particularly if the track lead had put two talks back to back because they were complementary, or had put them far apart because they were similar.