Each edition now has their own macros, so create a macros.html for the third edition too. Since the utterances theme differs between editions, we need to move it from snippets.html to the edition-specific macros.html too.
- Split macros.html in per-edition files
- Create new snippets.html macro file with utterances snippet
- Move support.html to snippets.html as new macro
- Create new `latest_post` macro in 2nd edition macros.html
