Small refactoring of Submission

Before tackling #1699 I'm trying to really understand the `Submission` model. I found some small refactorings along the way.

This PR tries to get closer to standard active record, and adds a test for the `#not_commented_on_by()` scope.

I *believe* the `submitted?` method on submission is not used anywhere, so I removed it.  The tests still pass. But I could be wrong in which case I should *not* remove it and instead add a test. Let me know!