media: Use ETag and If-None-Match

Note: this requires a reindex of the media database. For the default
CAS backend, use the following:

```bash
for i in */*/*/*/*.json; do
    etag="$(echo $i | sed -e 's/\///g' -e 's/\.json$//')";
    mv "$i" "$i.bak"
    cat "$i.bak" | jq '. + { "etag": '\""$etag"\"'}' > "$i"
    rm "$i.bak"
done
```

This change is backwards compatible, but caching headers won't be
emitted without etags present in the metadata.

Actual etags are backend-specific and might differ from backend to
backend.

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