Bot interaction: set an item priority override for a world

Allow the player to override the default item priority for a specific world.

Respond to a DM'd command of !item <world> <item> <priority>:

• Accept only "known" items for the world
• Accept only high, medium, and low priorities
• Send confirmation that the new item priority has been set

Respond with an error message if:

• The world is not "registered" to that user.
• The item is invalid
• The priority is invalid

On receiving command, save item priority override to world data file, then reload priority composition (either just for that item, or for entire world).

Next version(s) may instead use an assisted interaction flow, like:

• Player requests item priority change with abbreviated command (world and item only)
• Bot responds with reaction-tagged message to show available priorities
• Player selection of reaction sends "full" command to bot for processing
• Bot responds with confirmation that the new item priority has been set