One of my most sublime summer moments this year came on a gorgeous sunny day as I zipped along St. Paul's downtown riverfront, the warm wind whistling past my ears, on a motorized vehicle that made almost no noise.Yes, it was an electric scooter. No, it wasn't a Segway Human Transporter (good guess, though). I'll get to that upright contraption in a second.We're all conditioned to hear rumbling engines when moving on self-propelled vehicles, so the relative silence of an electric scooter is an unexpected delight.So many kinds of electric scooters are now on the market, at so many prices,...