Add games/stats endpoint

[iliana]

This adds a /v1/games/stats?game={gameId} endpoint which returns all the relevant statsheets for a game. It seems to be reasonably performant (in earlier seasons, this is aided by looking for the last statsheet before the next games would begin).

[iliana]

Looking for the last statsheet recorded before the next games begin means we don't return any statsheets that were backfilled, so I'm going to adjust that logic.