Switch to short GUIDs.

We talked previously about the possibility of using shorter, prettier GUIDs, like http://perma.cc/Q54Y-TKB9 instead of http://perma.cc/0zfQPLh5R8z . This patch does that, so we can see how it would work.

Existing Perma links will continue to work.

If the user accidentally uses the wrong case or leaves out the hyphen they'll be forwarded to the correct place. So g54ytkb9 will forward to Q54Y-TKB9.

Going down to 8 characters with all caps reduces us to about one trillion possible GUIDs, which seems like plenty. It is small enough, however, that we do have to check after generating one to be sure we don't have a collision -- so it no longer allows for completely uncoordinated generation between servers.