"It is frequently said that speech that is intentionally provocative and therefore invites physical retaliation can be punished or suppressed. Yet, plainly no such general proposition can be sustained. Quite the contrary.... The provocative nature of the communication does not make it any the less expression. Indeed, the whole theory of free expression contemplates that expression will in many circumstances be provocative and arouse hostility. The audience, just as the speaker, has an obligation to maintain physical restraint."