empathy: Fix judging interface to properly merge two entire groups

Previously, when selecting an item that was a part of a larger group,
it could get pulled out of that group to be merged with the other
selected item. This was absolutely bewildering to players.

Instead, the right thing is to entirely merge the two groups to which
each of the two selected items belong. That's what we do here, and
using Set objects rather than simply arrays as we had before.