seo: prevent crawling comment, show_seq links.

* Mark /comments/:id links in comment timestamps as nofollow to prevent
  Googlebot from crawling these links.
* Mark /posts/:id/show_seq links as disallowed in robots.txt to prevent
  Googlebot from crawling forward/back links on posts.

app/components/comment_component/comment_component.html.erb

@@ -24,7 +24,7 @@
         <% end %>
       <% end %>
     </div>
-    <%= link_to time_ago_in_words_tagged(comment.created_at), comment, class: "message-timestamp" %>
+    <%= link_to time_ago_in_words_tagged(comment.created_at), comment, class: "message-timestamp", rel: "nofollow" %>
   </div>
 
   <div class="content">

app/views/robots/index.text.erb

@@ -9,6 +9,7 @@ Disallow: /posts?tags=source:*
 Disallow: /posts?tags=ordfav:*
 Disallow: /posts?tags=approver:*
 Disallow: /posts/*/favorites
+Disallow: /posts/*/show_seq*
 
 Disallow: <%= artist_urls_path %>
 Disallow: <%= artist_versions_path %>