DISCORD_USERID_WHITELIST must be empty to register INTERACTIONS ENDPOINT URL

[BadToxic]

When adding the INTERACTIONS ENDPOINT URL for the discord application

I always got the notofication the URL could not be verified and the changes not be saved. When I checked the code I saw, that entries in DISCORD_USERID_WHITELIST prevent Discord's check requests from going through.

It would be good to at least mention in the readme, that the whitelist must be empty and can be set after registering the URL.

[jarylc]

Ah I didn't know DISCORD_USERID_WHITELIST affected the onboarding process, I'll see if there's a better way to resolve that.