Author: Mark Nelson
MDL-46574 gradereport_history: added report viewed event
Part of MDL-46191
http://git.moodle.org/gw?p=moodle.git;a=commit;h=6668a9046fbe51b2c6435c6e2c1859d9452c132a
http://github.com/moodle/moodle/commit/6668a9046fbe51b2c6435c6e2c1859d9452c132a
+ 2.8 en [eventgradereportviewed,gradereport_history]