fix(challenges): challenge csv export now has proper filename

This sets the Content-disposition filename for the Challenge CSV export
to be {challenge_id}.csv.

Fixes #2689.

src/controllers/challenges.js

@@ -98,6 +98,7 @@ api.csv = function(req, res) {
       })
       output.push(uData);
     });
+    res.header('Content-disposition', 'attachment; filename='+cid+'.csv');
     res.csv(output);
   })
 }