Author: Stephen Bourget
MDL-45837 Grades: Add events for grade letters
http://git.moodle.org/gw?p=moodle.git;a=commit;h=31647f0cc99b426405425fb968f045102c1603e3
http://github.com/moodle/moodle/commit/31647f0cc99b426405425fb968f045102c1603e3
+ 3.5 en [eventgradelettercreated,core_grades]
+ 3.5 en [eventgradeletterdeleted,core_grades]
+ 3.5 en [eventgradeletterupdated,core_grades]