user submission export

test plan:
* as a student, visit /dashboard/data_exports
* should be able to export a zip file containing
 all of the student's submissions

refs #CNVS-14242

Change-Id: Iaab216e7411c27fa6d259f677e40a75d3de3f5a8
Reviewed-on: https://gerrit.instructure.com/36872
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>