Use the correct logic for choosing which pieces to focus on when assigning them to someone

[Lingnik]

it looks like PuzzlePieceViewSet.get_random() is what the API is calling and needs to be updated to factor in the SQL query in findUnconfidentPuzzlePieces(), which I think should consistently find a batch of 100 "do these first" pieces, focusing on ones that are about to be finished first but ordered randomly.

This is very similar to #86, but actually fixing #86 as-written (which I think would mean ignoring that they have a solution?) would result in us focusing on a bunch of old ones in the near term that we probably don't care about as much right now.