It’s beginning to look like D.C. is a mecca for all the world’s build-your-own-salad and fro-yo creation chains, with Canadian-born Freshii joining the ranks with its first District location opening today. Freshii, at 2001 M St. NW, is adding its incarnation of the customizable food model with salads, burritos, soups and tart yogurt to the list of quick, casual eateries in Foggy Bottom. And what better way to welcome itself to the neighborhood than…