Author: Amaia Anabitarte
MDL-64400 core_messages: Allow cron to delete unread notifications too
http://git.moodle.org/gw?p=moodle.git;a=commit;h=da73f1364db2dcc0ee0d15b7350cbbbcf6223f12
http://github.com/moodle/moodle/commit/da73f1364db2dcc0ee0d15b7350cbbbcf6223f12
+ 3.5 en [configmessagingdeleteallnotificationsdelay,core_admin]
+ 3.5 en [messagingdeleteallnotificationsdelay,core_admin]