remove codelab.pdf file, add redirect; move graffle files to be under src/diagrams

The moves result in a huge, irrelevant diff. The only visible change is that the redirect works, as tested by going to http://tweaks.dart-lang.appspot.com/codelabs/web-ui-writer/codelab.pdf and getting the HTML version.

R=mcampione@google.com

Review URL: https://chromiumcodereview.appspot.com/21153003