Add whitelisting (combined with making green)

Modify the green command to be something like:
\join
which whitelists that username and gives it the appropriate colours based on their discord roles

Could also combine this with giving access to #minecraft, in which case the command should be something like \minecraft or \mc, however then only those who have the game can join the chat, whilst others might be interested in discussing it even if they don't have the game

The issue names no files, tests, or entry points. First resolve whether the command should only whitelist users or also grant access to #minecraft, then define the command and Discord-role colour behavior; done means the selected scope works without leaving the access rules ambiguous.

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