Some songs take us back to a moment. Others open a door to an entire history. When Starship’s “We Built This City” exploded into the Gen X soundtrack, Grace Slick seemed perfectly at home in the neon world of 1985. But follow her backward and the road leads through Jefferson Starship, “Find Your Way Back,” Jefferson Airplane, and finally to “White Rabbit” and the psychedelic heart of San Francisco. For this San Francisco girl, discovering that history meant realizing something wonderful: I thought Grace Slick had entered my musical world. It turned out I had entered hers.