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.

app/logical/user_name_validator.rb

@@ -24,7 +24,7 @@ class UserNameValidator < ActiveModel::EachValidator
       rec.errors.add(attr, "can't start with '#{name.first}'")
     elsif name =~ /[[:punct:]]\z/
       rec.errors.add(attr, "can't end with '#{name.last}'")
-    elsif name =~ /\.(html|json|xml|atom|rss|txt|js|css|csv|png|jpg|jpeg|gif|png|mp4|webm|zip|pdf|exe|sitemap)\z/i
+    elsif name =~ /\.(html|json|xml|atom|rss|txt|js|css|csv|png|jpg|jpeg|gif|png|avif|webp|mp4|webm|zip|pdf|exe|sitemap)\z/i
       rec.errors.add(attr, "can't end with a file extension")
     elsif name =~ /__/
       rec.errors.add(attr, "can't contain multiple underscores in a row")