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
Fix link formatting for /styleguide in README
Merge pull request #2126 from danbartlett/patch-1

Fix link formatting for /styleguide in README
Change button text from 'dismiss' to 'remove' and add a ? Fixes #2123
Merge pull request #2127 from danbartlett/dismiss
Clean up trailing whitespace
Mention Shopify as sponsors on the about page
Mention Rackspace as sponsors on the about page
Stores code and filename in a submissions solution
Merge pull request #2129 from LNA/save-incoming-solutions

Stores code and filename in a submissions solution
Delete unused file
Regenerate javascript