Add variant specific statistics

[zond]

Such as histograms of

• Solo winner nations
• Eliminated nations
• DIAS participating nations
• Score per nation

To make this less expensive, we also should decorate GameResult with the variant the game had, otherwise we first have to load all finished games with variant X then load the result for each game.

Adding that decoration, however, makes it necessary to backport it to make old results have it as well. Which requires a utility endpoint like _re_rate which makes the server load all GameResults with empty Variant, load their games, populate the Variant fields of the GameResults and then saves the GameResults again.