Add (undo) buttons onto page history, for people with rollback privilege only (that...

Add (undo) buttons onto page history, for people with rollback privilege only (that's changeable very simply.. just put in place to ward off whingers). Also, minorly, switch the undoafter and undo parameters if they're given the wrong way around, which seems like a more sensible thing to do than silently fail. I would have renamed them undo1 and undo2, but that'd cause all kinds of chaos.