Consider removing preview videos

[shish]

Do people actually use the preview videos?

• If yes, maybe we should replace "one full-size encode and two previews" with "two full-size encodes"
• If no, maybe we should delete the preview encodes

But first, let's add logging to see what is actually used...

[shish]

For the record, stats from Hotaru:

• 307 tracks played
• 240 tracks were sung without anybody watching the preview
• 77 tracks were sung after somebody watched the preview
• 96 tracks were not-sung after somebody watched the preview

[shish]

Preview-quality encodes ended up being useful for the low-powered podium tablet