feat(comments): replace disqus_identifier with comment_id

Disqus is not the only comment provider. We plan to add more comment providers.
Others too have the feature of providing custom id for the comments. Therefore, it does not make
sense to use disqus_identifier for them. comment_id variable can be used with any comment service.

templates/_includes/comments.html

@@ -13,7 +13,9 @@
     <div class="accordion-group">
         <div class="accordion-heading">
             <a class="accordion-toggle disqus-comment-count" data-toggle="collapse" data-parent="#accordion2"
-                {% if article.disqus_identifier %}
+                {% if article.comment_id %}
+                    data-disqus-identifier="{{ article.comment_id }}"
+                {% elif article.disqus_identifier %}
                     data-disqus-identifier="{{ article.disqus_identifier }}"
                 {% endif %}
                 href="{{ SITEURL }}/{{ article.url }}#disqus_thread",

templates/_includes/disqus_scripts.html

@@ -42,7 +42,9 @@
 <div id="disqus_thread"></div>
 <script type="text/javascript">
     var disqus_shortname = '{{ DISQUS_SITENAME }}';
-    {% if article.disqus_identifier %}
+    {% if article.comment_id %}
+        var disqus_identifier = '{{ article.comment_id }}';
+    {% elif article.disqus_identifier %}
         var disqus_identifier = '{{ article.disqus_identifier }}';
     {% else %}
         var disqus_identifier = '{{ SITEURL }}/{{ article.url }}';