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=57afd82ab0636f232d597d0c02f0b776410c4c8d
http://github.com/moodle/moodle/commit/57afd82ab0636f232d597d0c02f0b776410c4c8d
+ 3.7 en [configmessagingdeleteallnotificationsdelay,core_admin]
+ 3.7 en [messagingdeleteallnotificationsdelay,core_admin]