Add support for animated WEBP or AVIF emotes

[agg23]

The external emote providers tend to provide all of their emotes in WEBP or AVIF form. Neither of which is parsed properly by UIImage, particularly when animated. I attempted to use SDWebImageWEBPCoder and SDWebImageAVIFCoder, but they animated very strangely (often displaying blank frames for 30s at a time).

The 7TV API does not explicitly support GIFs, but we're using GIFs for now to bypass this issue. See 63f67c8