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/models.py

@@ -7,6 +7,7 @@ import pytz
 from django.core.exceptions import ValidationError
 from django.db import models
 from util.validate_on_save import ValidateOnSaveMixin
+from xmodule_django.models import CourseKeyField
 
 
 class MidcourseReverificationWindow(ValidateOnSaveMixin, models.Model):
@@ -17,7 +18,7 @@ class MidcourseReverificationWindow(ValidateOnSaveMixin, models.Model):
     overlapping time ranges.  This is enforced by this class's clean() method.
     """
     # the course that this window is attached to
-    course_id = models.CharField(max_length=255, db_index=True)
+    course_id = CourseKeyField(max_length=255, db_index=True)
     start_date = models.DateTimeField(default=None, null=True, blank=True)
     end_date = models.DateTimeField(default=None, null=True, blank=True)