More small refactorings of Submission

In preparation for #1699, some more refactoring the `Submission` class.

Visually differentiate activerecord scopes from other types of class methods by using the scope syntax
Replace `#exluding` scope with the identical `#not_submitted_by` scope.
Slightly reduce SQL string snippets
Beef up `#aging` test to make refactoring a little safer
Move not_commented_on_by test to test/exercism/... (where I realized I should have put it), instead of test/app/...