Redirect current users to dev channel

Our current users getting builds from trunk which equivalent to dev channel.
Since the old google cloud storage buckets will go away, we'll redirect the
current users to dev channel.
Update all links on dartlang.org to new google cloud storage buckets

Changes, apart from replacing links from the old buckets with links to
gs://dart-archive/, are:
 - replace integration links with links to stable channel
 - replace continuous links with links to dev channel
Merge pull request #87 from mkustermann/dartlang-gcslinks-to-channels

Redirect current users to new dev channel downloads + Changed links to stable channel on dartlang.org