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-2/extra.html

@@ -1,6 +1,6 @@
 {% extends "edition-2/base.html" %}
 
-{% import "macros.html" as macros %}
+{% import "snippets.html" as snippets %}
 
 {% block title %}{{ page.title }} | {{ config.title }}{% endblock title %}
 
@@ -17,6 +17,6 @@
     <hr>
     <section>
         <h2>Comments</h2>
-        {{ macros::utterances() }}
+        {{ snippets::utterances() }}
     </section>
 {% endblock after_main %}