Set `crossorigin="anonymous"` on the <img> tag for thumbnails. This
makes it so we don't send session cookies in requests for thumbnails.
Since images are served from cdn.donmai.us, and since session cookies
are set on *.donmai.us, session cookies are sent by default with every
thumbnail request.
Not sending cookies saves up to 1kb of overhead per thumbnail request
(in reality, it's less than this because of HTTP/2 multiplexing and
header compression).
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