Move macros

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.

blog/templates/snippets.html

@@ -9,13 +9,3 @@
     Thank you!
 </p>
 {% endmacro support %}
-
-{% macro utterances() %}
-    <script src="https://utteranc.es/client.js"
-        data-repo="phil-opp/blog_os"
-        data-issue-term="url"
-        data-label="comments"
-        crossorigin="anonymous"
-        async>
-    </script>
-{% endmacro utterances %}