Make course ids and usage ids opaque to LMS and Studio [partial commit]

This commit updates common/djangoapps.

These keys are now objects with a limited interface, and the particular
internal representation is managed by the data storage layer (the
modulestore).

For the LMS, there should be no outward-facing changes to the system.
The keys are, for now, a change to internal representation only. For
Studio, the new serialized form of the keys is used in urls, to allow
for further migration in the future.

Co-Author: Andy Armstrong <andya@edx.org>
Co-Author: Christina Roberts <christina@edx.org>
Co-Author: David Baumgold <db@edx.org>
Co-Author: Diana Huang <dkh@edx.org>
Co-Author: Don Mitchell <dmitchell@edx.org>
Co-Author: Julia Hansbrough <julia@edx.org>
Co-Author: Nimisha Asthagiri <nasthagiri@edx.org>
Co-Author: Sarina Canelake <sarina@edx.org>

[LMS-2370]

common/djangoapps/embargo/tests/test_models.py

@@ -1,13 +1,14 @@
 """Test of models for embargo middleware app"""
 from django.test import TestCase
 
+from xmodule.modulestore.locations import SlashSeparatedCourseKey
 from embargo.models import EmbargoedCourse, EmbargoedState, IPFilter
 
 
 class EmbargoModelsTest(TestCase):
     """Test each of the 3 models in embargo.models"""
     def test_course_embargo(self):
-        course_id = 'abc/123/doremi'
+        course_id = SlashSeparatedCourseKey('abc', '123', 'doremi')
         # Test that course is not authorized by default
         self.assertFalse(EmbargoedCourse.is_embargoed(course_id))