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/student/auth.py

@@ -35,7 +35,7 @@ def has_access(user, role):
         return True
     # if not, then check inferred permissions
     if (isinstance(role, (CourseStaffRole, CourseBetaTesterRole)) and
-            CourseInstructorRole(role.location).has_user(user)):
+            CourseInstructorRole(role.course_key).has_user(user)):
         return True
     return False
 
@@ -81,6 +81,6 @@ def _check_caller_authority(caller, role):
     if isinstance(role, (GlobalStaff, CourseCreatorRole)):
         raise PermissionDenied
     elif isinstance(role, CourseRole):  # instructors can change the roles w/in their course
-        if not has_access(caller, CourseInstructorRole(role.location)):
+        if not has_access(caller, CourseInstructorRole(role.course_key)):
             raise PermissionDenied