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/user_api/tests/factories.py

@@ -3,6 +3,7 @@ from factory.django import DjangoModelFactory
 from factory import SubFactory
 from student.tests.factories import UserFactory
 from user_api.models import UserPreference, UserCourseTag
+from xmodule.modulestore.locations import SlashSeparatedCourseKey
 
 # Factories don't have __init__ methods, and are self documenting
 # pylint: disable=W0232, C0111
@@ -18,6 +19,6 @@ class UserCourseTagFactory(DjangoModelFactory):
     FACTORY_FOR = UserCourseTag
 
     user = SubFactory(UserFactory)
-    course_id = 'org/course/run'
+    course_id = SlashSeparatedCourseKey('org', 'course', 'run')
     key = None
     value = None

common/djangoapps/user_api/tests/test_user_service.py

@@ -5,6 +5,7 @@ from django.test import TestCase
 
 from student.tests.factories import UserFactory
 from user_api import user_service
+from xmodule.modulestore.locations import SlashSeparatedCourseKey
 
 
 class TestUserService(TestCase):
@@ -13,7 +14,7 @@ class TestUserService(TestCase):
     """
     def setUp(self):
         self.user = UserFactory.create()
-        self.course_id = 'test_org/test_course_number/test_run'
+        self.course_id = SlashSeparatedCourseKey('test_org', 'test_course_number', 'test_run')
         self.test_key = 'test_key'
 
     def test_get_set_course_tag(self):