Show whether fields are saved on Quick Grader

!191 (merged) introduced a behaviour change to only save grades in the new Quick Graded, when the field loses focus (onBlur).
As it was pointed out, this onBlur event does NOT trigger if the user closes the webpage.

Therefore a visual feedback should be provided to the instructors doing the grading:

• When a grade changes, its border should become red (or similar visual feedback).
• When a grade is saved (upon focus lost), its border should become green (or similar visual feedback).

If a field gets focus, but the grade is not modified, then its border color should not be modified at all. (As it should not be saved in the first place.)