In API, support fetching most-frequent players

[shaldengeki]

Right now we have a field on a card that returns a list of players sorted by the raw number of times they played it. This list almost always shows dwarvintime at the top because they just have more games in our database. It would probably be more interesting to show, like, "bayesian-smoothed probability of play".

In the API, add a field that computes this and returns a list sorted by this.