Helm: smart rollback

The rollback:<environment> jobs print helm history, roll back to the last successful revision before the live one, and post a one-line Slack message with the revision numbers. They don't check that the target revision is healthy first, and the message doesn't say which chart and app versions changed.

Items

• Validate the target revision is healthy before rolling back.
• Show the chart and app version before and after in the Slack message.