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
Reviewers: @ormsbee, @sarina, @brianhw, @rocha