SE-2451 Ensure courseware models allow unicode rendering

[pomegranited]

Follow-up PR for https://github.com/edx-olive/edx-platform/pull/20

Fixes similar issues with courseware models.

Testing instructions

Deploying to courses.campus.gov.il

1. Create a user with a Hebrew username, e.g. this user (<-- ok to delete).
2. From the shell, ensure the user has a related courseware.models.XModuleStudentPrefsField record (user above already does)

   from courseware.models import XModuleStudentPrefsField
   from django.contrib.auth import get_user_model
   jamal = get_user_model().objects.get(username='גמאל_חטיב_1')
   XModuleStudentPrefsField.objects.create(student=jamal, value=u'"en"', module_type=u'video', field_name=u'transcript_langauge')

3. As a superuser in Django Admin, delete the user created in step 1.

Author Notes & Concerns

1. This PR only fixes the courseware.model unicode rendering, which is a common type of associated record for active users. With the production users, there may be more models that need to be fixed.
2. Will need to be ported to Hawthorn/Ironwood, but will not be required in Juniper since python3 format strings support unicode by default.

Reviewer

• @pkulkark

[pkulkark]

@pomegranited LGTM 👍

• I tested it: Verified that the user mentioned in the testing instructions can be deleted successfully.
• I read through the code.
• I checked for accessibility issues
• Includes documentation

[itsjeyd]

For future reference, we didn't create an upstream PR for this change because:

... this fix isn't applicable to master anymore, since all Python3 strings support unicode. But we'll need to port this fix to Hawthorn and/or ironwood depending on Campus's upgrade path.

Cf. comment.