4188 Validation to prevent kudos-ing your own work

https://code.google.com/p/otwarchive/issues/detail?id=4188

While we don't show the Kudos button on a user's own work, there are other ways to give yourself kudos. Here we add a check that the user leaving kudos is not the user who created the work. (The error message assumes the item is a work, but I used `self.commentable_type.classify.constantize.find_by_id()` so this would theoretically work with other types of commendable items, if we ever add kudos to anything else.)