Pomelo support

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Discord is changing how usernames work on their platform. This is internally codenamed "Pomelo".

I haven't taken a deep look into how Calamity handles discriminators or usernames, or any potential logic related to looking up users by their usernames. Here's some important notes, though:

• Pomelo-migrated users have a discriminator of "0". I'm not sure if this field will be outright removed after migration completes.
• Users have an nullable global_name field, which corresponds to the "Display Name" field in the User Profile Settings.
• Precedence as to what "name" a user has when being displayed to a human is: guild nickname > display name/global_name > username.

See: discord.py patch