Automatic cleanup of downloads

**Is your feature request related to a problem? Please describe.**
Downloads hang around in the database forever, along with their generated data on the server's disk. While download numbers are overseeable, this poses a problem for long-term storage space. From a privacy standpoint, it's also a good idea to remove old downloads.

**Describe the solution you'd like**
A timed task that checks all downloads for expiration. For all expired downloads, remove database object and associated files.
Expiration time should be discussed among team.