appview,knotserver,camo: add AVIF and modern image format support #251

Add support for AVIF, JPEG XL, and HEIF image formats across the stack. Currently these formats fail at three points in the rendering pipeline:

Knot server returns 403: http.DetectContentType() in Go doesn't recognize AVIF/JXL/HEIF, returning application/octet-stream, which hits the default case and returns HTTP 403.
Camo proxy returns 415: image/avif, image/jxl, and image/heif are missing from the allowed MIME types list in mimetypes.json.
Blob viewer doesn't render as image: .avif, .jxl, .heic, .heif are missing from the image extension switch in NewBlobView.

knotserver/xrpc/repo_blob.go: Replace the single if for .svg with a switch that also overrides MIME types for .avif, .jxl, .heic, .heif
camo/src/mimetypes.json: Add image/avif, image/heif, image/jxl to the allowed MIME types list
appview/repo/blob.go: Add .avif, .jxl, .heic, .heif to the image extension case in the blob viewer

Any repo with .avif images in its README, e.g. https://tangled.org/@overby.me/overby-me

AVIF has 97%+ browser support and is increasingly common as a modern replacement for JPEG/PNG/WebP.

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