When it comes to fashion, whether it’s shoes, clothing, or bags, there are always two sides to the story. On the one hand there are the official fashion trends that descend on us like pronouncements from Mount Olympus. They filter downward from designers and gurus until they wind up, years later, in the bargain bins at department stores. On the other hand is the real-world trends that have more to do with attractive utility than anything else. The hottest trend in the world of bags right now isn’t coming out of Paris or Milan. It’s coming from the streets of…