posts index: clean up limit:<n> / order:random parsing (#2894)

* Move the limit:<n> / order:random metatag parsing from the controller
  to the post set.

* Introduce `Tag.has_metatag?` and use it to parse these metatags
  instead of using a regex (#2894).

app/views/posts/partials/index/_posts.html.erb

@@ -19,7 +19,7 @@
     </div>
   <% end %>
 
-  <% unless @random.present? %>
+  <% unless post_set.is_random? %>
     <%= numbered_paginator(post_set.posts) %>
   <% end %>
 </div>