Download Submission doesn't seem to care about non-staff users

People can mark their submissions and public download, but it looks like the download view doesn't permit downloads if you're not a staff member:

• https://git.fossgalaxy.com/iggi/Comet/blob/master/fg_competitions/views.py#L32

this should probably be changed to (is a staff member) OR (is an authenticated user AND the track allows downloads AND the submitter allows public download)