ziafazal/api-calculate-progress-on-completions: calculate progress on module completion

@mattdrayer  @chrisndodge I have added `progress` app having two model `StudentProgress` and `StudentProgressHistory` to save progress on  post_save event of `CourseModuleCompletion` entry. Added `generate_progress_entries.py` management command to generated progress entries for existing data. Please review. 
Are we no longer returning 'position' in the response if the value is None?  If so we should probably have @martynjames take a look on the client side.