If we deliberately create a Codereview assignment and *want* to permit submissions to the original assignment after submitting here, we can't

[blerner]

This is currently by design, since it would invalidate the self-eval's relevance to the current submission. We should fix this, so that a self-eval refers to a specific submission, and then resubmitting to the underlying assignment should make the self-eval not be a UsedSub, but it shouldn't be prohibited outright....