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
I would pull this to after the check on the feature flag. It doesn't make sense to define this variable if it is unused. I think
```python
if settings.FEATURES.get("CREATIVE_COMMONS_LICENSING", False):
    fields['license'] = request.json.get('license')
```
would work, no?