Smart playlists required

[jace]

Our current approach for automatic playlists -- especially when tagging speakers -- is inefficient for queries from the video page. To find a list of speakers or count how many users have liked a video, we have to query for all playlists containing this video, then look at the type tag on each playlist.

This approach is going to cause severe performance issues as user activity grows, so we should fix it before launch with a new approach to auto playlists: smart playlists

Step A:

1. Remove like/unlike for now. It'll come back in a different form later since binary states (like or dislike) are not suitable for our content
2. Remove the "speaking-in" auto playlist. Add a many-to-many relationship between Video and Channel named "speakers". Let speakers be tagged directly to a video.

Step B:

1. The path /<channel>/speaking-in renders channel.videos, which is the reverse side of the relationship Video.speakers. It renders using the same playlist template.