Fix copy plugins

the low level method `CMSPlugin.copy_plugin` messes up the reference to the old plugin changing it to the new one
This has no consequence if the old plugin instance is not "reused", but it may affect lower level use (like in blueprint)
not strictly related, but isn't this a bit over-fuzzy? when would this ever cause 0 queries? (and on a completely unrelated note, how the heck does Page.publish cause 200+ queries?!)