In Marketplace, soft-delete all apps, even incomplete ones (bug 942108)

https://bugzilla.mozilla.org/show_bug.cgi?id=942108

This fix the weird IntegrityError happening when we delete an incomplete app with only deleted versions. 

@cvan do you think this is the right call ? My reasoning is, since we are soft-deleting even versions in the Marketplace, I think it makes better sense to soft-delete apps in all cases, even incomplete ones. This could generate extra garbage in the db though. The alternative is to make sure django properly deletes attached versions when we really delete an app.