fix thumbnail generation on every push

post-receive git hook is also tweaked. since thumbnails aren't needed on
site build time, they can be created after by copying `thumbs.sh` script
into site art directory, running it and deleting afterwards.

`hooks/post-receive`
```

{
    cp "$TMP_GIT_CLONE/art/thumbs.sh" "$PUBLIC_WWW/art/thumbs.sh"
    cd "$PUBLIC_WWW/art"
    sh thumbs.sh
    rm thumbs.sh
}

```

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