Trigger an Event on authorized discount

* Triggers an event (that can be found on Adminitrators>Alerts)
  when a user log in to apply an greater discount.

* Fix for some rollback problems with ALLOW_HIGHER_DISCOUNT

Fixes: http://bugs.async.com.br/show_bug.cgi?id=5617

Change-Id: I968c4c634a1281960976a186ed945d6a8cc6dac6