Add `is_staff?` to syllabus cache key.
Add `is_staff?` to syllabus cache key.

Closes #625.

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