Bot interaction: set a world context

To simplify interactions, respond to a DM'd command of !world <world> to set which world future commands will apply to. Also accept a bare !world command to reset the world context to nothing.

The bot will need to track a "state" for each player (at the player/user level, not at the world level). Stub an initial state manager to use in-memory only (which will be lost on each bot backend restart).

Respond with a confirmation that the world context has been set. Respond with an error if the user is not "registered" to the given world.

Prepend any command responses with the world affected. Alter !status command (#11 (closed)) and !item command (#12 (closed)) to not require the world in the command, but require a world context to be set.