Replace WebP previews with AVIF

[japsu]

https://netflixtechblog.com/avif-for-next-generation-image-coding-b1d75675fe4

Pre-requisites:

• Unflagged AVIF support in browsers (Chrome 85, Firefox 87)
  • https://caniuse.com/#search=avif
• AVIF support in Pillow or similar straightforward Python encoding & metadata solution
  • Add AVIF plugin (decoder + encoder using libavif) python-pillow/Pillow#5201