feat(tazjin/blog): Add distinction between published & updated times

Fixes a long-standing todo by adding an additional field for the time
at which a post was updated, and handling this in both site
generation (to note the update date after the publish date) and in
Atom feed generation (by populating both the updated and published
fields).

Change-Id: If80db43459b2a0c77eea4dde7255651b5d6cd64b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2766
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/homepage/feed.nix

@@ -14,7 +14,7 @@ let
     title = post.title;
     content = readFile (renderMarkdown post.content);
     published = post.date;
-    updated = post.date; # TODO(tazjin): this should be distinct from published
+    updated = post.updated or post.date;
 
     links = singleton {
       rel = "alternate";