Fixing some issues in partnership report, the fixes are listed bellow

[jonasporto]

• set completed_at order in notifier, because this isn't set before, the report wasn't working correctly
• add new skip before filter in payment_controller, was blocking auto conciliation transactions for Iugu gateway
• add order by desc created_at in buying report to facilitate search for new buys
• created_at new migration for fill completed_at in all orders that are completed until now