refactors debate's tags related utility methods

tags are already loaded, using them instead of extracting them again for each debate saves 50 query per index (2 query per debate in the page)

app/models/debate.rb

@@ -86,13 +86,15 @@ class Debate < ActiveRecord::Base
   end
 
   def tag_list_with_limit(limit = nil)
-    tags.most_used(limit).pluck :name
+    return tags if limit.blank?
+
+    tags.sort{|a,b| b.taggings_count <=> a.taggings_count}[0, limit]
   end
 
   def tags_count_out_of_limit(limit = nil)
     return 0 unless limit
 
-    count = tags.count - limit
+    count = tags.size - limit
     count < 0 ? 0 : count
   end
 

app/views/shared/_tags.html.erb

@@ -3,7 +3,7 @@
 <% if debate.tags.any? %>
   <span class='tags'>
     <% debate.tag_list_with_limit(limit).each do |tag| %>
-      <%= link_to sanitize(tag), debates_path(tag: tag) %>
+      <%= link_to sanitize(tag.name), debates_path(tag: tag.name) %>
     <% end %>
 
     <% if debate.tags_count_out_of_limit(limit) > 0 %>