Update copyright headers (#3086)

Added and updated copyright headers.

Added some missing Flow types.

Removed an invalid prop-types import.

scripts/generateHeadingIDs.js

@@ -1,3 +1,7 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ */
+
 const fs = require('fs');
 const GithubSlugger = require('github-slugger');