Allow to exclude some combinations of users

[damienalexandre]

Sometimes Secret Santa is run in organisations where some people should not give each other a gift (think a family where a couple don't want to be assigned one to another) but still want to be part of the gift exchange.

We could allow users to select "forbidden assignations", when all the users are selected.

It should not be a preview of the draw, just a list of rules, to keep the magic / the secret!