Firefox users can not easily add search engines to the browser unless
the page serves an OpenSearch description. This CL adds said
description according to the documentation:
https://developer.mozilla.org/en-US/docs/Web/OpenSearch
Change-Id: I358c5940304f4abd9e45dd72a64e46d3ce44b3e3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3105
Tested-by: BuildkiteCI
Reviewed-by: cynthia <cynthia@tvl.fyi>
