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The search index will change when any content changes and the client
side search JS references the index file URL. So if we hash the index
file, that changes the hash of the JS that references the new name,
which in turn, changes the JS `script` tag for all HTML pages, which
means we have to upload new versions of every HTML page when content
anywhere on the site changes.

Not that big of a deal, but I'd rather deploys not have to upload and
invalidate every page on the site when unrelated stuff changes.

So instead use a stable address for the search index URL and have it be
a resource that we ask the browser to always revalidate (like for the
HTML content itself) instead of allowing a long cache lifetime.