empathy: Don't let any player give kudos to themself

Which would not be very sporting of course, so we don't allow it.

Note that this restriction is applied after the majority-ruled
matching. So a player can't even give kudos to a word they didn't
submit and then hope to get matched into it.

It's also worth noting that the group can decide to split a group
where one player gave kudos. That will mean that both sides of the
split get kudos from the player, which seems to match the kudos-giving
player's intent.