Fix for static student gradebook alignment

Fix for https://moodle.org/mod/forum/discuss.php?d=261626 - instead of reducing #user-grades down to 2px which affects table regardless of the core setting 'grade_report_fixedstudents' to bring into alignment, this is now set at 0px such that the scroll bar at the top is completely adjacent as the bottom one is.  Then reducing the specific '.left-scroller' (the padding on the contained table has no effect) down by 2px brings everything into alignment when 'grade_report_fixedstudents' is set.