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/django_comment_common/tests.py

@@ -1,5 +1,6 @@
 from django.test import TestCase
 
+from xmodule.modulestore.locations import SlashSeparatedCourseKey
 from django_comment_common.models import Role
 from student.models import CourseEnrollment, User
 
@@ -21,13 +22,13 @@ class RoleAssignmentTest(TestCase):
             "hacky",
             "hacky@fake.edx.org"
         )
-        self.course_id = "edX/Fake101/2012"
-        CourseEnrollment.enroll(self.staff_user, self.course_id)
-        CourseEnrollment.enroll(self.student_user, self.course_id)
+        self.course_key = SlashSeparatedCourseKey("edX", "Fake101", "2012")
+        CourseEnrollment.enroll(self.staff_user, self.course_key)
+        CourseEnrollment.enroll(self.student_user, self.course_key)
 
     def test_enrollment_auto_role_creation(self):
         student_role = Role.objects.get(
-            course_id=self.course_id,
+            course_id=self.course_key,
             name="Student"
         )