added leaderboard features

[sarge1989]

Issues

• Resolves #307

Summary

• Main concern is to avoid calculating the entire leaderboard on request
  • Basically, we will instead store and update score, duration, and number of votes/correct votes backend, and update them during the relevant events
  • On request, the leaderboard will be sorted by score and extracted. Thus the only operation is a sort, rather than calculating everyone's score.
• Added quite some data fields to both checker and voteRequest to facilitate the above
• Replaced achievements page with leaderboard page