appview,knotserver,camo: add AVIF and modern image format support
Add support for AVIF, JPEG XL, and HEIF image formats across the stack:
- knotserver/xrpc: override MIME types for formats that Go's
http.DetectContentType does not recognize (.avif, .jxl, .heic, .heif),
using a switch statement like the existing .svg override
- camo: add image/avif, image/heif, and image/jxl to the allowed MIME
types list
- appview/repo: add .avif, .jxl, .heic, .heif to the image extension
list in the blob viewer
Without these changes, AVIF files (and other modern formats) are rejected
by the knot server with a 403 (detected as application/octet-stream),
blocked by the camo proxy with a 415, and not previewed in the blob viewer.
Signed-off-by: Niclas Overby <niclas@overby.me>