Minimal grade CSV downloads via S3

Do not merge -- pull not ready yet. Here for collaboration purposes.

@sarina: tag

UX added to Beta Instructor Dashboard!

What a course staff member will see: http://d.pr/i/zl0z

What an edX employee (is_superuser) will see: http://d.pr/i/Kk4i

The functional difference here is that edX employees are allowed to generate new grade reports, but course staff can only download old ones. This is intended to be temporary, to allow us to control load on the system. It is governed by the new feature flag `ALLOW_COURSE_STAFF_GRADE_DOWNLOADS` - if `ALLOW_COURSE_STAFF_GRADE_DOWNLOADS` is set to True, then all course staff and instructors will have access to the grade report generation button.

Note that course staff will be able to see the status of any pending grade report instructor task in teh Pending Instructor Tasks table.