Author: Sara Arjona
MDL-64985 message: add aria label to cancel selection button
Add aria labels for cancel edit message button (the little cross
when selecting messages).
http://git.moodle.org/gw?p=moodle.git;a=commit;h=13fe002e8ea57a82af00d0f8566b21d6c15fb373
http://github.com/moodle/moodle/commit/13fe002e8ea57a82af00d0f8566b21d6c15fb373
+ 3.7 en [cancelselection,core_message]