Mark US pints as common, rather than Imperial ones

Historically, pints were defined in terms of gallons. KUnitConversion marks the US gallon as common, but not the US pint. As a result, if you're in the US and enter "1 pint" or "1 gallon" you'll get inaccurate measurements of the other one. This is a somewhat incoherent UX.

While technically the UK still uses the Imperial gallon for some things, the country is officially on Metric. As a result, I think it makes sense to define the US pint as common, given that it's still an official unit of measurement in the USA.

This commit does that.

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