Make tau configurable by admin

[jauggy]

Tau is configurable by admin in Site Config -> Lobby section

The default value of tau is unchanged (which is 1/3)

Also adding in prevent_sigma_increase to true

[L-e-x-o-n]

Can you please put it under a new Openskill section?

[jauggy]

I've added it to a Rating section. Calling it more generic just in case we change from openskill to another system.

Also I've added in prevent_sigma_increase to true so that we don't have stuff like this where uncertainty increases:

This occurs when tau is very high. The way it works is that before a match is rated, the value of tau is added to uncertainty then the match is rated, so if tau is really high you can gain uncertainty.

    new_rating_groups =
      Enum.map(rating_groups, fn team ->
        Enum.map(team, fn {mu, sigma} ->
          {mu, Math.sqrt(sigma ** 2 + options.tau ** 2)}
        end)
      end)

    output = options.model.rate(new_rating_groups, options)

So your before the match gets rated, your uncertainty becomes Math.sqrt(sigma ** 2 + options.tau ** 2)
Then the match gets rated, and there's a chance uncertainty rises if it doesn't fall enough after rating.

The prevent_sigma_increase is already in the openskill lib.