Delete files after user downloads them

**Problem**: Files are left on the server after the user downloads them. This is particularly problematic when they download the entire submission set. On the production server, the 2018 set reached 92% disk space, and it filled it once the user attempted to download them all. I will be increasing the disk space on production after this round is complete, but the **app/static/data/archive/** folder need to be empty most of the time (i.e., after each download, delete the file that was created) to solve the root cause.

**Solution**: https://stackoverflow.com/questions/24612366/flask-deleting-uploads-after-they-have-been-downloaded