srgb Submission Download Dialog

fixes CNVS-11500
fixes CNVS-11549

test plan:
 - in srgb, select an assignment
 - click the 'Download all submissions' button
 - a dialog should appear that should say it is zipping the file
 - after it zips, it should download the file immediately
 - closing the dialog should cancel the download
 - reopening the dialog should restart the download

Change-Id: I0c312ec352f8ffa5d39977aab901e8873e1156fb
Reviewed-on: https://gerrit.instructure.com/31275
Reviewed-by: Liz Abinante <labinante@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>