4851: Poster can revoke ship-its


A really convenient feature would be to allow the owner of a review to revoke ship-its against that review. This would be useful in a situation where someone has given a review a ship-it, and then the owner of the review adds a new diff or updates the review in some way. The previous ship-it could possibly be invalid, because its context is now old. By being able to revoke the ship-it, we can mark it as expired due to new context.