MDL-31262 fix quiz review capability check.
MDL-31262 fix quiz review capability check.

This change is logically the same as changing
    $this->has_capability('mod/quiz:viewreports')
to
    $this->has_capability('mod/quiz:viewreports') || $this->has_capability('mod/quiz:preview')
in the original code. However, I rewrote the logic so that we test the
most common and performance-critical case first - that of a student
lookign at their own attempt.

dilawar/moodle