Wind speed is displayed in unfamiliar units (knots)

Since gnome-weather#10 (closed) we display wind speed information, e.g. "Southwest / 8.0 knots":

This leads me to wonder: what on Earth is a knot? In the US, wind speed should be displayed in miles per hour. It would be better to not display wind speed at all than to display it in unfamiliar units.

I'm about to do a late 3.36.0 release, and it's long past string freeze, so displaying wind speed in mph isn't an option for 3.36. We can (a) defer wind speed to 3.38 and figure out how to display the right units there, or (b) show wind speed in 3.36 only if celsius is the selected temperature unit.

For 3.38, we can decide: do we want to add a separate option for wind speed unit? Or tie it to the temperature unit?

CC @ebassi