feat: support leaf bundles for blog posts, refactor header images

content/blog/2020-03-system-hackers-report-lyon.md

@@ -7,8 +7,9 @@ tags:
   - fsfe
   - report
   - server
-headerimage: /blog/syshackers-group-hacking.jpg
-headercredits: Hackers in their natural working environment. For the picture we took off the black ski masks and gloves.
+headerimage:
+  src: /blog/syshackers-group-hacking.jpg
+  text: Hackers in their natural working environment. For the picture we took off the black ski masks and gloves.
 
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