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/middleware.py

@@ -5,6 +5,7 @@ Adds user's tags to tracking event context.
 from track.contexts import COURSE_REGEX
 from eventtracking import tracker
 from user_api.models import UserCourseTag
+from xmodule.modulestore.locations import SlashSeparatedCourseKey
 
 
 class UserTagsEventContextMiddleware(object):
@@ -19,6 +20,7 @@ class UserTagsEventContextMiddleware(object):
         course_id = None
         if match:
             course_id = match.group('course_id')
+            course_key = SlashSeparatedCourseKey.from_deprecated_string(course_id)
 
         context = {}
 
@@ -29,7 +31,7 @@ class UserTagsEventContextMiddleware(object):
                 context['course_user_tags'] = dict(
                     UserCourseTag.objects.filter(
                         user=request.user.pk,
-                        course_id=course_id
+                        course_id=course_key,
                     ).values_list('key', 'value')
                 )
             else: