TaxIt/TaxRateSetting

Now there isn't contract breaking, I think it might be worth trying the simplest version, which is just let players arbitrarily set the tax rate each turn for each toll booth, between 0 and the limit for that toll booth.

I don't think taxes from a toll booth should apply to goods being shipped to the city controlling the booth.

I'm unsure if they should apply to goods being shipped from production spots controlled by the city. I can see arguments either way.