Support "fixed" metadata for edge posts

Allow setting an "fixed: 2022-10-16" metadata line to edge blog posts to mark the post as fixed in the edge blog overview and the post detail page. For old posts it's also supported to set it to a boolean as "fixed: yes"

I've updated the metadata for the existing edge posts. The fixed-ness state of the last few posts is guessed as in the screenshot above

Closes #146