Allow default grader assignments

[david-yz-liu]

Currently, instructors must assign graders to students on a per-assignment basis, often using the same settings from one assignment to the next. Instructors have requested the ability to set up a "default" grader assignment, either on a per-course basis or on a per-assignment basis but before groups have been created. This would simplify the workflow for instructors when managing the grading.

Some examples are:

• Assign graders to specific sections
• Configure random grader assignment
• Always assign all graders to all students