Fix resetting between competitions

[ChandlerSwift]

When ending one competition and beginning another, even when the database is cleared, it is possible that remnants of an old competition will linger. The one that bit us last competition is that the gold container wasn't rebuilt, so the calc competition was testing against jas's gold!

This should definitely be documented, and preferably we should add some more detailed checks when starting a competition. Perhaps this could take the form of hashing gold like we do users' programs? We already have a hash_dir function; this would also help for when we change gold like in #45.