count_submissions_need_grading called even if user cannot grade the assignment

Tracker issue created from forum post: https://moodle.org/mod/forum/discuss.php?d=412662

During periods of high load when users are logging in, the function "count_submissions_need_grading" is causing the most load on our database server.

The function is called from mod_assign_core_calendar_provide_event_action but returning the count of submissions needing grading is returned even if the user does not have the capability of grading that item.

{{LINE 1549: mod_assign_core_calendar_provide_event_action: /mod/assign/lib.php}}
{{ if ($event->eventtype == ASSIGN_EVENT_TYPE_GRADINGDUE) {}}
{{     $name = get_string('grade');}}
{{     $url = new \moodle_url('/mod/assign/view.php', [}}
{{         'id' => $cm->id,}}
{{         'action' => 'grader'}}
{{     ]);}}
{{     $itemcount = $assign->count_submissions_need_grading();}}
{{     $actionable = $assign->can_grade($userid) && (time() >= $assign->get_instance()->allowsubmissionsfromdate);}}
{{ } else}}

The query used in $itemcount is expensive for MySQL to be run as it isn't using indexes in some parts, causes full table scans and also uses 2 temporary tables.

A better option would be checking if the user can grade the item or if it is actionable before counting submissions, and if not, assign "$itemcount = 0;" instead.