Update disqus embed to use {{comment_id}}

refs TryGhost/Ghost#8760

- disqus code must use `{{comment_id}}` instead of `{{id}}` from 1.0.0 onwards.

post.hbs

@@ -80,7 +80,7 @@ into the {body} of the default.hbs template --}}
                 <script>
                     var disqus_config = function () {
                         this.page.url = '{{url absolute="true"}}';
-                        this.page.identifier = 'ghost-{{id}}';
+                        this.page.identifier = 'ghost-{{comment_id}}';
                     };
                     (function() {
                         var d = document, s = d.createElement('script');