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=f10c2934aaf0e105aa49b97e77f9a1b2c5f23db8
http://github.com/moodle/moodle/commit/f10c2934aaf0e105aa49b97e77f9a1b2c5f23db8
+ 3.6 en [configmessagingdeleteallnotificationsdelay,core_admin]
+ 3.6 en [messagingdeleteallnotificationsdelay,core_admin]