Add the performer to the UserRightsChanged hook

Add the performer to the UserRightsChanged hook

This allows extensions (e.g. Echo) to detect who made the change without
relying upon $wgUser. It also allows for differentiation between
autopromotion entries which will pass in `false` as the performer.

Change-Id: Idebd78b54dcea1bdc84c83f402e87b240ab4ade1