Add ui for accessing search post counts

fixes #2528

app/views/counts/posts.html.erb

@@ -0,0 +1,16 @@
+<div id="c-counts">
+  <div id="a-posts">
+    Post count for <%= link_to params[:tags].present? ? params[:tags] : "/posts", posts_path(:tags => params[:tags]) %>:
+    <% if @count == (Danbooru.config.blank_tag_search_fast_count || 1_000_000) %>
+      timed out
+    <% else %>
+      <%= @count %>
+    <% end %>
+  </div>
+</div>
+
+<% content_for(:page_title) do %>
+  Post Counts - <%= Danbooru.config.app_name %>
+<% end %>
+
+<%= render "posts/partials/common/secondary_links" %>

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

@@ -10,5 +10,7 @@
     <li><%= link_to "Random post", random_posts_path(:tags => params[:tags]), :id => "random-post" %></li>
 
     <li><%= link_to "Mobile version", mobile_posts_path(:tags => params[:tags]) %></li>
+
+    <li><%= link_to "Post count", posts_counts_path(:tags => params[:tags]) %></li>
   </ul>
 </section>