Allow to exclude items from being drawn

[lucaa]

Currently, it is possible to re-draw someone that was already drawn and canceled because they didn't claim the prize.

While this can be handy (depending on the rules about how the prizes are to be claimed, showing up later and having a chance to re-drawn may be interesting), it can also pollute the pool of choices with invalid ones and make the raffle complex in terms of length and relevance.

As a new feature, it should be possible to exclude an item from the pool of choices (upon cancel).