So -- you like Second Saturdays? Do you like the way this local arts night has morphed into a street fair? Have you watched the event grow so big that you've hoped it could take over the central city more than once a month? Your prayers may be answered this month when a new Saturday street fair debuts in Sacramento. Sponsored by Sacramento News & Review (SN&R), "Design Downtown" will bring mostly locally made or "do it yourself" (DIY) fashion and art to the K Street Mall from 3 to 9 p.m. every third Saturday starting July 18. "We looked at the success of Second Saturday and said, 'Maybe that's something we can build on,' " said Clay Nutting, the sales and marketing manager for the paper