Fix sort column names in search query

lib/search.rb

     sort =
       case params[:sort]
       when 'thread'
-        'threads.created_at DESC'
+        'created_at DESC'
       when 'post'
-        'posts.created_at DESC'
+        'last_post_created_at DESC'
       else
-        'threads.created_at DESC'
+        'created_at DESC'
       end
     search_threads(db, query, username, sort, offset)
   when 'posts'