Remove files from s3 when classroom session ends

[sumanyu]

List of updates

• Major update is cron job that runs every 6 hours - finds active classroom sessions and for each marks them as inactive and deletes all shared files from s3 server
• Deletes files from s3 server when users both end session within 6 hours (explicitly)

  To do later

• Figure out a way to add privacy to files such that only the tutor and tutee associated with a particular session can access to files

[davinwong]

cleaning up sessions along with the files at the same time is great