participate: Use a custom model
The way we were keeping the document id completely broke down with a recent change in Ideascube: ideascube!905 In addition, this was wrong anyway, because we were using a custom form to keep the attribute when creating the message, but not when editing it. And publishing a message means editing it, so de facto those attributes were all always lost. So we need a custom model, to keep track of the linked media. This should probably be an N-to-N relationship directly in Ideascube, but this is not the time. This also makes the cleaning command much nicer!