Fix manage.py to ouput the help of the django command if requested

Commands like the following were not working correctly:
```
$ python manage.py lms runserver --lms
```
Fix error when there is an invalid YouTube property in a video module
Add command to dump the course_ids available to the LMS
Add command to dump the structure of a course as a JSON
Add LMS command to dump a course to stdout as a tar.gz file