MDL-46565 gradereport_*: renamed event string names

MDL-46565 gradereport_*: renamed event string names

This makes the strings more descriptive and consistent.

AMOS START
  MOV [eventreportviewed, gradereport_grader],[eventgradereportviewed, gradereport_grader]
  MOV [eventreportviewed, gradereport_outcomes],[eventgradereportviewed, gradereport_outcomes]
  MOV [eventreportviewed, gradereport_overview],[eventgradereportviewed, gradereport_overview]
  MOV [eventreportviewed, gradereport_user],[eventgradereportviewed, gradereport_user]
AMOS END