Tiny refactoring of UUID setting

Remove tiny duplication of the functionality of Exercism.uuid
Explicitly return `true` from before_create callback as per:

http://stackoverflow.com/questions/4735141/rails-3-do-i-need-to-give-return-true-in-a-before-save-callback-for-an-object
and
http://blog.teksol.info/2010/09/28/unintented-consequences-the-pitfalls-of-activerecord-callbacks
Merge pull request #2117 from bluerogue251/tiny_uuid_refactoring

Tiny refactoring of setting user's uuid key
Remove reference to current user in stats - fixes #2113
Small refactoring of Submission
Merge pull request #2119 from bluerogue251/submission_refactorings

Small refactoring of Submission
Merge pull request #2118 from code-R/2113-remove-current-user-usage-in-stats

Remove reference to current user in stats - fixes #2113
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
Rename language to track_id

Rename the `language` column in the user_exercises and submissions
tables to `track_id`, since 'language' had been a misnomer for what was
actually a track_id.

Add getter and setter methods that substitute #track_id for #language.
Eventually, when all references to `language` are all updated to be
references to `track_id` in the main code, then these temporary methods
will be able to be removed. The temporary methods are in:
`lib/exercism/temporary_language_to_track_id_translator.rb`

Do a *bare minimum* mass renaming of `language` to `track_id`. Bare
minimum means just enough to get all the tests back to passing.
Eventually, we'll go through the codebase in more detail and rename all
the affected places so that the temporary methods mentioned above are no
longer necessary.