Add staticfiles dirs to devstack settings.

To handle static files for working with XML courses on devstack.
@jarv 
Cleaner syntax with `path.py`:
```py
(course_dir, DATA_DIR / course_dir)
for course_dir in DATA_DIR.listdir()
if (DATA_DIR / course_dir).isdir() and
    not (DATA_DIR / course_dir / 'static').isdir()
```