Calls fail if avatar is not externally hosted

[nathonius]

If avatars are hosted on the foundry server, the calls to the discord API fail. For example:

{
  "username": "Gamemaster",
  "avatar_url": "http://icons/svg/mystery-man.svg",
  "content": "",
  "embeds": [{ "title": "", "description": "Rolled 1d20, and got a 1 = 1" }]
}

400 Bad Request
{
  "avatar_url": [
    "Not a well formed URL."
  ]
}

Obviously http://icons/svg/mystery-man.svg is not a valid URL.

EDIT: Oop, looks like this might already have been fixed? I see it in the changelog anyway.

Ah, looking at the code it uses the invite URL as its base, smart. Going to close this since setting that fixes it, this just needs to be documented.