avatar_url_as improvements

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. :\

discord/utils.py

@@ -281,3 +281,7 @@ def sane_wait_for(futures, *, timeout, loop):
 
     if len(pending) != 0:
         raise asyncio.TimeoutError()
+
+def valid_icon_size(size):
+    """Icons must be power of 2 within [16, 1024]."""
+    return ((size != 0) and not (size & (size - 1))) and size in range(16, 1025)