The submissions API has a get_score() method which, given a student item,
will look up the relevant ScoreSummary and return the latest Score it
contains. Our data download should do the same.

To avoid confusion, we have elected to hide scores that could not be
returned from api.get_score(). The other options are:
- include a score that may be for a different submission for the same
    student item, if using ScoreSummary.latest in all cases
- include scores that may not "count", because they'll never be the
    return value of api.get_score()

Test has been updated.