add attachment export feature to dashboard

[hcientist]

Assuming #14 works, there will be cells that have filenames referencing the students' recordings they uploaded as mp3s, but we need to actually provide those mp3s for export as well.

• update the template to have a link (to a new, as yet non-existent view) that when clicked will respond with a zip file containing the mp3 files from the dataset showed in the template.