Author: Amaia Anabitarte
MDL-63279 messaging: Add admin setting for toggling site-wide messaging
http://git.moodle.org/gw?p=moodle.git;a=commit;h=fdb45f7943643a0ebc93b5b50df57468cc833597
http://github.com/moodle/moodle/commit/fdb45f7943643a0ebc93b5b50df57468cc833597
+ 3.6 en [configmessaging,core_admin]
+ 3.6 en [configmessagingallusers,core_admin]
+ 3.6 en [messagingallusers,core_admin]