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/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 %>