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Review: integer math

Numeric code gets reviewed for four things: overflow, division by zero, signed/unsigned mix-ups, and empty input.

Your turn: this pull request adds stats for the match history page. The specification:

win_rate(wins, games): whole-number percentage, rounded down; 0 when there are no games. It must work for career totals in the billions.
average(scores): the mean as a double; 0.0 for no scores. Scores can be large.
last_n_total(scores, n): the sum of the last n scores (all of them if there are fewer than n).

There are four bugs.