FIRST AUTHOR'S PREMIERE: Principals in the world's first "author's premiere," staged last night at the Michigan Theater amid much lighthearted fanfare, are shown standing outside the theater before the ceremony got under way. Pictured from left to right are Valentine Davies, Hollywood scriptwriter; George J. Burke, local attorney; Mayor William E. Brown, jr.; and Shirley W. Smith, retired University vice-president whose short story became the basis for the movie, "It Happened Every Spring," given its first public showing here last night. (others pictures on page 28)