fix: if comment count enabled, show only on supported pages

Don't attempt to show Disqus comment count on page types that do not have corresponding Disqus comment block.

Currently, just Post and Page types support commenting.

See #1206

layouts/partials/page_metadata.html

@@ -49,7 +49,7 @@
   </span>
   {{ end }}
 
-  {{ $comments_enabled := and site.DisqusShortname (not (or site.Params.disable_comments (eq $page.Params.comments false))) }}
+  {{ $comments_enabled := and site.DisqusShortname (in (slice "post" "page") $page.Type) (not (or site.Params.disable_comments (eq $page.Params.comments false))) }}
   {{ if and $comments_enabled (site.Params.comment_count | default true) }}
   <span class="middot-divider"></span>
   <a href="{{ $page.RelPermalink }}#disqus_thread"><!-- Count will be inserted here --></a>