[yokhizjo] Chess960 opening selector

Let an online Chess960 interface preferentially choose initial positions that both players have not played, or played least frequently.  There remains a tricky detail of balancing the two players concerns: probably minimize the maximum of the two frequencies.

A player could game the system by being able to predict in advance what initial position will be chosen and prepare for it, negating the benefit of Chess960.  Need to have some randomness so that the optimal initial position is not always chosen.

Offer special recognition to players who have played every initial position with both colors, and have won from every initial position.