Support APNG for Banners, Icons, etc.

GIFs are really showing their age with its compression and size, which really emphasizes a need for allowing APNG uploads for any animated content.

APNGs support full image quality and are smaller in size, whereas GIFs almost always end up with visible compression artifacts. This is most apparent in animated server banners, where you're only allowed 8MB to fill in a full-size banner; having to deal with GIF compression in the process.

Stickers already support this, so why not allow it for the rest of the application?