static_format will only apply to static (not animated) avatars. Makes
it easier to grab gif-or-'format' of an avatar. Defaults to 'webp'
This is for a similar usecase to avatar_url_as(format=None), except
one can specify the non-animated format, instead of always using
webp.
add User.avatar_is_animated property.
add validation for avatar_url_as, since invalid arguments result in
a url which will return 415, which can be confusing for a user. (They
just see a blank page)
Discord accepts size=16-2048, but images cap at 1024px, so accept 16-1024
Discord accepts "jpg", "jpeg", "png", "gif", and "webp", *unless* the
avatar is not animated, in which case "gif" is not supported. :\
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