Feature/issue 20 leaderboard

Fixed the leaderboard to match SoW description: Only show Top 10
Postgres is terrible at counting, it's _seriously_ slow. Iirc this is because of its MVCC nature and as a result often needs to do a table scan to eliminate tombstones in order to do a full count. Once this site hits 100K+ users performance is going to tank. 

If we're going to need this often can we find a way of doing this without requiring Postgres to do a `count()`? Can we get away with an incrementing participants integer on a location somewhere?