add a PlayerRating model

[pmrowla]

PlayerRating should be used to track a player's goonskill rating numbers, as well as the player's total RWS and number of rounds played. Having all of this info in one table will make it easier to do the queries we need on the SM plugin side.

[pmrowla]

Also, I think that average RWS should be calculated using the harmonic mean instead of the arithmetic mean. This reduces the impact of outlier rounds, and rewards players who are more consistent than others.

[pmrowla]

harmonic mean won't work since there are 0 values. I tried geometric mean and everyone's numbers end up being super low, so I guess I'll stick with arithmetic