Restructure macros

- 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

blog/templates/edition-1/section.html

@@ -1,6 +1,6 @@
 {% extends "edition-1/base.html" %}
 
-{% import "macros.html" as macros %}
+{% import "edition-1/macros.html" as macros %}
 
 {% block title %}{{ section.title }} | {{ config.title }}{% endblock title %}