related tags: refactor /related_tags.js to avoid updating unchanged columns.

Refactor the /related_tags.js call so that when the "Related tags"
button is clicked, it only replaces the columns that actually changed,
not the entire related tags section.

This avoids rebuilding the Recent / Frequent / Translated Tags / Artist
columns every time the "Related tags" button is clicked.

app/views/posts/partials/show/_edit.html.erb

@@ -89,5 +89,5 @@
     <%= submit_tag "Submit", :class => "ui-button ui-widget ui-corner-all" %>
   </div>
 
-  <%= render "related_tags/container" %>
+  <%= render "related_tags/container", related_tags: nil %>
 <% end %>