Fixed some bugs in tasks, apple feature:

* Update ordergroup stats when task is destroyed.
* Removed assigned caching attribute in task object.
* A lot of eager loading for tasks controller.

app/views/tasks/_archive_tasks.html.haml

@@ -14,6 +14,4 @@
       %tr
         %td= task.due_date unless task.due_date.nil?
         %td= link_to "#{task.name} (#{task.duration}h)", :controller => "tasks", :action => "show", :id => task
-        %td
-          - unless task.users.empty?
-            = task.users.map(&:nick).join(", ")
+        %td= task_assignments task