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_groups/models.py

@@ -2,6 +2,7 @@ import logging
 
 from django.contrib.auth.models import User
 from django.db import models
+from xmodule_django.models import CourseKeyField
 
 log = logging.getLogger(__name__)
 
@@ -23,7 +24,8 @@ class CourseUserGroup(models.Model):
 
     # Note: groups associated with particular runs of a course.  E.g. Fall 2012 and Spring
     # 2013 versions of 6.00x will have separate groups.
-    course_id = models.CharField(max_length=255, db_index=True,
+    # TODO change field name to course_key
+    course_id = CourseKeyField(max_length=255, db_index=True,
                                  help_text="Which course is this group associated with?")
 
     # For now, only have group type 'cohort', but adding a type field to support