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/course_modes/models.py

@@ -9,6 +9,8 @@ from collections import namedtuple
 from django.utils.translation import ugettext as _
 from django.db.models import Q
 
+from xmodule_django.models import CourseKeyField
+
 Mode = namedtuple('Mode', ['slug', 'name', 'min_price', 'suggested_prices', 'currency', 'expiration_datetime'])
 
 class CourseMode(models.Model):
@@ -17,7 +19,7 @@ class CourseMode(models.Model):
 
     """
     # the course that this mode is attached to
-    course_id = models.CharField(max_length=255, db_index=True)
+    course_id = CourseKeyField(max_length=255, db_index=True)
 
     # the reference to this mode that can be used by Enrollments to generate
     # similar behavior for the same slug across courses