Add Extra Credit property for Grade Item

Add Extra Credit property for Grade Item in Gradebook Setup

User Interface Changes

UI changes for this piece of functionality will be incorporated into the effort to build the hierarchical table tag.

API Changes

A boolean property will need to be added to the Assignment class and GradeableObject hibernate mapping for the sub-class Assignment.

Calculations will need to be changed. Note that these changes will in many cases happen in the same code as the changes for "Drop Lowest".

We will want to coordinate these changes with

• Add "Equal Weight" functionality requires API changes
• Add Drop Lowest functionality requires API changes