On save, create a versioned copy of an article

[csarven]

Where applicable, and if the UI permits, saving the article should create a versioned (timestamped) copy. The canonical URL of the article should point (like a symlink or redirect) to the latest version. Similar to how W3C does it with the recommendations.

This issue should coordinate with another issue on persistent copies and identifiers (add issue number here) that are not necessarily HTTP IRIs.