Add support for "Extra Credit" in gradebook

Currently, and assignment with a maximum value of 10 will not permit an earned point value of 11. Instead, the message, "The grade 11 for <student name> is not valid." This makes it impossible to allow "extra credit" assignments, where it is possible to earn more than 100 percent.

Instead, the first time an attempt is made to enter more than the maximum point value, a dialog should appears that asks: "Allow extra credit on this assignment? Yes/No" If the user (teacher) answers "Yes", then a flag is set on that assignment to permit extra credit for all students on that assignment.