“It is not a minor detour on the tourist path nor a mere question of convenience, like deciding to enter through the garage door rather than trek around to the vestigial front porch. The closing of the front doors of the Supreme Court, like so many mindless decisions attributed to security concerns, is a grand affront — architecturally, symbolically, politically. The decision will enforce new and unwanted meanings on one of the city’s most dramatic and successful public buildings.”