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. 
See my other comment below -- I think it might be better to push DETACHED_CATEGORIES into a setting, versus making it a global/constant