fix user::get_avatar_decoration_url() and parsing of the field

[Commandserver]

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The endpoint for the avatar decoration seems to have changed: https://discord.com/developers/docs/reference#image-formatting-cdn-endpoints.

Same for the json structure: https://discord.com/developers/docs/resources/user#user-object-user-structure.

fix #1418

[netlify]

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