Add back the image for external links

Also remove download-orig.html.

Staged at:
https://links-dot-dart-lang.appspot.com/tools/download.html

BUG= https://github.com/dart-lang/www.dartlang.org/issues/1139, https://github.com/dart-lang/www.dartlang.org/issues/1126
R=shaza@google.com

Review URL: https://chromiumcodereview.appspot.com/719943002