Rework coop leaderboards to show only the best time for each player

If players grind the same mission over and over to improve their time, then each of their times stays in the leaderboard. So it can happen that the same person has the number 1, 2, 3, etc spots which is IMO a little strange for a leaderboard. It would make more sense to just keep the best time for each player or group of players and ignore all of their slower times.

This might require some database changes, I'm not entirely sure. I think the coop leaderboard table may be a little bit funky right now.