Decks for particular players, given once per world when they join.

Put a YDK deck list (as YGOPro, EDOPro, YGOPRODeck or the deck box's Export button write it) into a
folder named after the player's UUID:

    config/jadm/join_decks/<uuid>/<deck name>.ydk

or name the file itself after the UUID: config/jadm/join_decks/<uuid>.ydk. The player gets a deck box
named after the file, holding the cards themselves (they can take them out like any deck box's cards).
Each file is given once per world; a file added later is given on the player's next join, and
/jadm joindecks give <player> gives all of a player's decks again.
