World command: complete, forfeit

A broader application of item ignore support (see #56 (closed)): batch-set (or set a world-level flag) to skip sending all messages about items for a given world. Continue to log item receive counts and hint requests (#70 (closed)).

As subcommand: !world ignore

e.g.:

• World has to be forfeited for whatever reason
• World is completed, but has nothing remaining of any value