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 `clone` method.
unset_pks function
Merge branch 'clone-page'