THE PRO FOOTBALL ISSUE: The game is bigger than ever, and the price of membership is going up. The NFL has admitted Atlanta for $9 million, the AFL has accepted Miami for $7.5 million. But amid the plenty there is a shortage of prime quarterbacks. Tex Maule tells how Coach Norman Van Brocklin's young Vikings might surprise Green Bay in the West and, with Morton Sharnik, scouts all the NFL clubs in a special section illustrated with color photographs of the stars. Edwin Shrake analyzes the AFL teams. George Plimpton, who has pledged allegiance to the Detroit Lions, writes of the superfan's fearful joys. Plus the weekly news and features.