AWS: Optimize DB query for submission information.

Avoid n queries per submission to fetch submission result, token, files, etc,
by preloading them in the same query. Load time for a page with 1000
submissions goes from 30 seconds to about 1 second. Similar results with ES and
SS when they go to check on ALL THE SUBMISSIONS.
AWS: Improve performance of rankings page.
cmstestsuite: update to manage Datasets

We don't actually test datasets, but just put everything into the default
dataset.
SpoolExporter: update to manage Datasets
YamlLoader: update to manage Datasets
ContestExporter/Importer: change the --light behavior

And benefit from SubmissionResults and UserTestResults.