Generalize grade report to allow display of any report

We would like to be able to generate arbitrary reports and expose them to the instructor by simply uploading them to S3.  The existing grade download infrastructure pretty much supported that already, however, all of the internal and external structures were referring to the reports as exclusively grading related.

Fixes: AN-590
Merge pull request #2827 from mulby/gabe/report-downloads

Generalize grade report to allow display of any report