Don't show partially uploaded files on the /col view.

Resolves #561. The local list of files is compared with the submissions list in
the database to remove files which are currently being uploaded
but which have not yet completed and added to the databases.

It will probably be necessary to implement some kind of clean
up mechanism for file uploads don't complete as they will no longer
show up for deletion on the journalist interface. 
This works for now. I think generally it would be more elegant to avoid checking the file system for documents, and would instead be better to just use the databse, and give the database a more complete picture of what's going (i.e., teach it about Replies too).