Refactored inclusion of disqus script

- Added ARIA setting `aria-live="polite"` to comment output
- Removed duplicate <div id="disqus_thread"> from page/post templates
- Moved blocking JavaScript loading to the end of the document body.
- Merged JavaScript code for index and pages/posts using conditionals
- Moved global JavaScript disqus variables into the anonymous function to stop pollution of global namespace

.themes/classic/source/_layouts/post.html

@@ -17,10 +17,10 @@ single: true
     {% endunless %}
   </footer>
 </article>
-{% if site.disqus_short_name and page.comments != false %}
+{% if site.disqus_short_name and page.comments == true %}
   <section>
     <h1>Comments</h1>
-    <div id="disqus_thread">{% include post/disqus_thread.html %}</div>
+    <div id="disqus_thread" aria-live="polite">{% include post/disqus_thread.html %}</div>
   </section>
 {% endif %}
 </div>