Hide association documents when unavailable

One-sentence description

The year report of 16-17 is showing, but there is no file to back it. This results in an error when someone tries to access it.

Current behaviour

The year report of 16-17 is showing, but there is no file to back it. This results in an error when someone tries to access it.

Expected behaviour

There should not be an icon if there is no year report yet. It should be possible not to have reports. Note that it's a filefield, so setting null=True won't do much; blank=True should be enough to work with.

Steps to reproduce

1. Go to the documents pgae
2. Try to view the '16-'17 year report.