More small refactorings of Submission

Differentiate activerecord scopes from other types of class methods
Replace `#exluding` scope with the identical `#not_submitted_by` scope
Slightly reduce SQL string snippets
Beef up some tests to make refactoring a little safer
Move not_commented_on_by test to exercism/submission_test
Merge pull request #2121 from bluerogue251/submission_refactoring

More small refactorings of Submission