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/reverification/tests/factories.py

@@ -5,6 +5,7 @@ from reverification.models import MidcourseReverificationWindow
 from factory.django import DjangoModelFactory
 import pytz
 from datetime import timedelta, datetime
+from xmodule.modulestore.locations import SlashSeparatedCourseKey
 
 
 # Factories don't have __init__ methods, and are self documenting
@@ -13,7 +14,7 @@ class MidcourseReverificationWindowFactory(DjangoModelFactory):
     """ Creates a generic MidcourseReverificationWindow. """
     FACTORY_FOR = MidcourseReverificationWindow
 
-    course_id = u'MITx/999/Robot_Super_Course'
+    course_id = SlashSeparatedCourseKey.from_deprecated_string(u'MITx/999/Robot_Super_Course')
     # By default this factory creates a window that is currently open
     start_date = datetime.now(pytz.UTC) - timedelta(days=100)
     end_date = datetime.now(pytz.UTC) + timedelta(days=100)