Creative Commons Licensing for edX

This PR adds Creative Commons Licensing to the Open edX Platform.

Review tickets
T&L https://openedx.atlassian.net/browse/TNL-195
UX https://openedx.atlassian.net/browse/UX-948
wait, should you be checking here too if the course is licensable, eg 
```python
if settings.FEATURES.get("CREATIVE_COMMONS_LICENSING", False) and course.licenseable:
```
?