Use StudenData.grades_for_assignment in syllabus.
Use StudenData.grades_for_assignment in syllabus.

Use StudenData.grades_for_assignment in syllabus instead of executing a
query, making a trip to the database.

Closes #457.

mkoon/gradecraft-development,mkoon/gradecraft-development,zqian/gradecraft-development,peterkshultz/gradecraft-development,peterkshultz/gradecraft-development,mkoon/gradecraft-development,UM-USElab/gradecraft-development,zqian/gradecraft-development,UM-USElab/gradecraft-development,zqian/gradecraft-development,peterkshultz/gradecraft-development,UM-USElab/gradecraft-development,UM-USElab/gradecraft-development