Bot interaction: request notification of an item send

A player may be blocked until receiving a certain item. As item sends increase in volume, it is less clear when the blocking item is received.

Like #12 (closed), requires state tracking. More important to persist for this, but less important than #18 (closed). Track notification requests per-world.

Respond to a DM'd command !notify <item> when the given item is received (respect world context as in #13 (closed)):

• Send a confirmation message that the user will be notified

In ItemComposer, check for an outstanding notification request for the received item, send via DiscordBotDMOutputAdapter, then clear the notification request.