Cloning for WidgyPages

Puts a 'Clone' button on the widgy mezzanine WidgyPage admin (next to 'View on site'). Clicking it will ask you for a new title and create a new page with a cloned version of the widgy content, including the
working copy and history.

Cloning of the VersionTracker itself is done through its new `clone` method.
This makes querying commits inconsistent, we do `commits__reviewedversioncommit__approved_by__isnull=False` other places.