This was previously all inside of my personal homepage configuration,
but that's not really where it belongs.
This moves the blog post -> feed entry logic to //web/blog and moves
some other minor logic (like entry order) into the atom feed
implementation itself.
Change-Id: Idde0241c48e979580de73f2b9afd04e6ca7f4c9a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3770
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>