Copyedit pass on download pages
Also changed the archive page's name & location (slightly)
Some changes are to Dart code (and thus to the generated JS files)

Staged at:
https://archive-dot-dart-lang.appspot.com/tools/download_archive
(redirects to https://archive-dot-dart-lang.appspot.com/tools/download-archive/)
https://archive-dot-dart-lang.appspot.com/tools/download.html

BUG= https://github.com/dart-lang/www.dartlang.org/issues/1084
R=shaza@google.com

Review URL: https://chromiumcodereview.appspot.com/667793002