Reduce complexity: replace case statement with method lookup.

Created by: robbkidd

Code climate gave NoteObserver a "B" because of the complex notify_team_of_new_note method that used a case statement. According to straight code analysis, this change halves the complexity of the method.

A concrete benefit of this change is that it reduces the number of code changes needed to add an email notification to a Note on a new model--or, say, Snippet with no email currently implemented. One would only have to make changes to Notify (add a note_snippet_email method and view) and update the tests for this method. No further change needed within the method itself.