Make course ids and usage ids opaque to LMS and Studio [partial commit]

This commit adds all of cms.

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]

cms/djangoapps/contentstore/features/transcripts.py

@@ -165,8 +165,7 @@ def remove_transcripts_from_store(_step, subs_id):
     """Remove from store, if transcripts content exists."""
     filename = 'subs_{0}.srt.sjson'.format(subs_id.strip())
     content_location = StaticContent.compute_location(
-        world.scenario_dict['COURSE'].org,
-        world.scenario_dict['COURSE'].number,
+        world.scenario_dict['COURSE'].id,
         filename
     )
     try: