Rename "opponent" in both the interface and the code

Rename "opponent" in both the interface and the code

The code was incorrect, since what was called "opponent_info" was
really just a list of other players (some of which could be on the
same team rather than an opposing team) so the new name of
"other_players" is more accurate.

In the UI, we also now use "another player" instead of "opponent" to
just be more friendly I suppose.