This only adds the feed generation functions, but does not yet wire it
up to the blog content.
This was implemented against
https://validator.w3.org/feed/docs/atom.html and I've validated some
generated example feeds with the W3 validator.
Change-Id: Ide3ea90d3fa935047506aa87169100c2ead21284
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1709
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
916dba43e7
