Lazy loading of CourseMode model's internationalization.

The use of `ugettext` here instead of `ugettext_lazy` can cause cyclical imports if this module is loaded earlier than expected. Per the Django documentation:
https://docs.djangoproject.com/en/dev/topics/i18n/translation/#lazy-translation

And yes, this merge request is happening because it happened to us and left a very cryptic 'cannot import name' error. :)

* Partner information: 3rd party-hosted open edX instance
* Merge deadline: would be nice to have it merged by Nov 3rd to minimize code drift, but not an edX partner so no hard requirement