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/external_auth/views.py

@@ -576,9 +576,8 @@ def course_specific_login(request, course_id):
        Dispatcher function for selecting the specific login method
        required by the course
     """
-    try:
-        course = course_from_id(course_id)
-    except ItemNotFoundError:
+    course = student.views.course_from_id(course_id)
+    if not course:
         # couldn't find the course, will just return vanilla signin page
         return _redirect_with_get_querydict('signin_user', request.GET)
 
@@ -595,9 +594,9 @@ def course_specific_register(request, course_id):
         Dispatcher function for selecting the specific registration method
         required by the course
     """
-    try:
-        course = course_from_id(course_id)
-    except ItemNotFoundError:
+    course = student.views.course_from_id(course_id)
+
+    if not course:
         # couldn't find the course, will just return vanilla registration page
         return _redirect_with_get_querydict('register_user', request.GET)
 
@@ -934,9 +933,3 @@ def provider_xrds(request):
     # custom XRDS header necessary for discovery process
     response['X-XRDS-Location'] = get_xrds_url('xrds', request)
     return response
-
-
-def course_from_id(course_id):
-    """Return the CourseDescriptor corresponding to this course_id"""
-    course_loc = CourseDescriptor.id_to_location(course_id)
-    return modulestore().get_instance(course_id, course_loc)