Support toggling notifications for new hints

Similar to enabling DM notifications for items, allow players to enable DM notifications when receiving a hint for their world. Notification DMs should only be sent if:

• The hint is new - item+location pair hasn't been hinted before; and
• The hinted item+location pair has not already been found

Add a subcommand of #71 (closed), as !hint notify, to toggle the hints.shouldNotify flag in the active world context.