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/embargo/models.py

@@ -13,14 +13,17 @@ file and check it in at the same time as your model changes. To do that,
 from django.db import models
 
 from config_models.models import ConfigurationModel
+from xmodule_django.models import CourseKeyField, NoneToEmptyManager
 
 
 class EmbargoedCourse(models.Model):
     """
     Enable course embargo on a course-by-course basis.
     """
+    objects = NoneToEmptyManager()
+
     # The course to embargo
-    course_id = models.CharField(max_length=255, db_index=True, unique=True)
+    course_id = CourseKeyField(max_length=255, db_index=True, unique=True)
 
     # Whether or not to embargo
     embargoed = models.BooleanField(default=False)
@@ -42,7 +45,8 @@ class EmbargoedCourse(models.Model):
         not_em = "Not "
         if self.embargoed:
             not_em = ""
-        return u"Course '{}' is {}Embargoed".format(self.course_id, not_em)
+        # pylint: disable=no-member
+        return u"Course '{}' is {}Embargoed".format(self.course_id.to_deprecated_string(), not_em)
 
 
 class EmbargoedState(ConfigurationModel):