Author: Gilles-Philippe Leblanc
MDL-47501 gradebook: Add an indication that the grade letters was overridden
http://git.moodle.org/gw?p=moodle.git;a=commit;h=af14ef657a6ed5ffd1de6c9ff1eefecb6d018bd4
http://github.com/moodle/moodle/commit/af14ef657a6ed5ffd1de6c9ff1eefecb6d018bd4
+ 2.9 en [gradeletteroverridden,core_grades]