Users can see who updated a performance last.

@christopherThorsen, @GarthGoldwater - Can you take a look at this?  
But you're not FINDING it using the column, right? You're finding it by ID. The reason we index foreign keys is because `has_many`'s find records using those IDs. Correct me if I'm wrong, but I don't think that you're finding these records BY who last updated them.