Uniquify, part 2

This is the changes to actually use the uniquified value.

This doesn't uniquify all that much: mostly just the same person literally submitting exactly the same thing again (since it's sensitive to essentia build sha, tags, etc.). But I think this is better than throwing out submission number 6 unconditionally!

A future improvement would probably be to have the JSON document get split up: at least to separate the essentia information as one document with its own sha, the tags as their own, and the actual data. So probably at least six: lowlevel, tonal, rhythm, metadata/audio_properties, metadata/version, metadata/tags. Then if any of those is the same as a previous version already seen, it's just an integer foreign key away. Less split would probably be to have the three metadata ones plus an "everything else" that they're merged into, and more split would probably also split out individual subkeys of lowlevel/tonal/rhythm. Anyway, the goal would be to go for something that isn't too much complexity to build but still gives us good savings on space without us needing to throw out data.