fix(web/blog): make base URL configurable for atom feeds

This was previously hardcoded to tazj.in, which is not going to work
of course.

Instead it now takes the blog config which has a new baseUrl
parameter. For ease of use, the configs of my and the TVL blog have
been moved into a location that is accessible in the tree for reuse.

Change-Id: I94e71aaa7859db4380eb7013740a17f6b6a02620
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3777
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

users/tazjin/homepage/feed.nix

@@ -22,7 +22,7 @@ let
     };
   });
 
-  allEntries = (map blog.toFeedEntry depot.users.tazjin.blog.posts)
+  allEntries = (with depot.users.tazjin.blog; map (blog.toFeedEntry config) posts)
              ++ (map pageEntryToEntry pageEntries);
 
   feed = {