uploads: add support for uploading .avif files.

Features of AVIF include:

* Lossless and lossy compression.
* High dynamic range (HDR) images
* Wide color gamut images (i.e. 10- and 12-bit color depths)
* Transparency (through alpha planes).
* Animations (with an optional cover image).
* Auxiliary image sequences, where the file contains a single primary
  image and a short secondary video, like Apple's Live Photos.
* Metadata rotation, mirroring, and cropping.

The AVIF format is still relatively new and some of these features aren't well
supported by browsers or other software:

* Animated AVIFs aren't supported by Firefox or by libvips.
* HDR images aren't supported by Firefox.
* Rotated, mirrored, and cropped AVIFs aren't supported by Firefox or Chrome.
* Image grids, where the file contains multiple images that are tiled
  together into one big image, aren't supported by Firefox.
* AVIF as a whole has only been supported for a year or two by Chrome
  and Firefox, and less than a year by Safari.

For these reasons, only basic AVIFs that don't use animation, rotation,
cropping, or image grids can be uploaded.

test/files/avif/README.md

@@ -0,0 +1,7 @@
+Test file sources:
+
+* https://github.com/link-u/avif-sample-images
+* https://github.com/AOMediaCodec/av1-avif/tree/master/testFiles
+* https://github.com/AOMediaCodec/libavif/tree/main/tests/data
+* https://colinbendell.github.io/webperf/animated-gif-decode/avif.html
+* https://0xc0000054.github.io/pdn-avif/using-image-grids.html