Fix author URLs

All the authors now have either a null URL, a valid URL or one on the
Internet Wayback Machine.

Where it seemed likely (say because the domain no longer exists) emails
were set to null also.

Now unless a bug is found in the script this really should finish the
checks of authors.html.

author/Paul_Heckbert.json

@@ -5,7 +5,7 @@
     "full_name" : "Paul Heckbert",
     "sort_word" : "heckbert",
     "location_code" : "US",
-    "email" : "ph@cs.cmu.edu",
+    "email" : "heckendo@uidaho.edu",
     "url" : "https://www.cs.cmu.edu/~ph/",
     "alt_url" : null,
     "deprecated_twitter_handle" : null,