[Gradebook] Display both grade scheme letter and numerical mark in override column

[JarrodHiles]

Currently, the override column will only show either the numerical value or the letter grade corresponding with the course grading scheme.

Displaying the grade scheme letter is helpful for students to see in their progressive mark but from the instructors point of view, once they have entered numerical values into the override column, they have to export the gradebook if they want to check the underlying value in future.

Given that the override column by nature supersedes the Total column, which shows both percent and letter grade, it would be beneficial for the override column to show both.