post versions: add ability to search post versions by tags.

* Add ability to search /post_versions by added tags, removed tags, or
  changed tags (added or removed).

* Add 'History' link to the sidebar of the /posts index. This is a
  shortcut for a /post_versions search of the current tag.

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

@@ -13,9 +13,10 @@
     <% end %>
 
     <li><%= link_to "Deleted", posts_path(:tags => "#{params[:tags]} status:deleted"), :rel => "nofollow" %></li>
-
     <li><%= link_to "Random", random_posts_path(:tags => params[:tags]), :id => "random-post", :rel => "nofollow", :"data-shortcut" => "r" %></li>
-
+    <% if Tag.is_simple_tag?(params[:tags]) %>
+      <li><%= link_to "History", post_versions_path(search: { changed_tags: params[:tags] }) %></li>
+    <% end %>
     <li><%= link_to "Count", posts_counts_path(:tags => params[:tags]) %></li>
   </ul>
 </section>