MDL-46936 gradereport_grader: Fixes for AJAX editing on the grade report

RTL support for keyboard navigation.
New function for editing_cancelled.
Fixed CSS style which cause the feedback field to extend over the next cell.
Removed broken TAB code from non-edit mode.

This issue is a part of the MDL-46658 Task.
This issue is a part of the MDL-25544 Epic.