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/user_api/models.py

@@ -2,6 +2,8 @@ from django.contrib.auth.models import User
 from django.core.validators import RegexValidator
 from django.db import models
 
+from xmodule_django.models import CourseKeyField
+
 
 class UserPreference(models.Model):
     """A user's preference, stored as generic text to be processed by client"""
@@ -44,7 +46,7 @@ class UserCourseTag(models.Model):
     """
     user = models.ForeignKey(User, db_index=True, related_name="+")
     key = models.CharField(max_length=255, db_index=True)
-    course_id = models.CharField(max_length=255, db_index=True)
+    course_id = CourseKeyField(max_length=255, db_index=True)
     value = models.TextField()
 
     class Meta:  # pylint: disable=missing-docstring