Self-deleting progress should keep assignments with top_submission_status

[felipecalvo]

If a user self-deletes their progress and re-does some exercises, they'll be awarded experience again, as if they had never done them in the first place.

If instead of destroying assignments we soft delete them, keeping top_submission_status, we also get the benefit that students can practice by redoing exercises without actually falling out of an organization's requirements, such as, say, do # number of exercises or a certain %. (We should communicate this clearly if we go forward with this idea).

And, of course, they won't be able to farm experience (though I know it's actually harmless) as we use top_submission_status to calculate it.

[luchotc]

This is a really good observation!