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

This commit adds updated requirements.

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]

requirements/edx/github.txt

@@ -17,7 +17,7 @@
 -e git+https://github.com/appliedsec/pygeoip.git@95e69341cebf5a6a9fbf7c4f5439d458898bdc3b#egg=pygeoip
 
 # Our libraries:
--e git+https://github.com/edx/XBlock.git@cfe5c37f98febd9a215d23cb206a25711056a142#egg=XBlock
+-e git+https://github.com/edx/XBlock.git@fc5fea25c973ec66d8db63cf69a817ce624f5ef5#egg=XBlock
 -e git+https://github.com/edx/codejail.git@71f5c5616e2a73ae8cecd1ff2362774a773d3665#egg=codejail
 -e git+https://github.com/edx/diff-cover.git@v0.2.9#egg=diff_cover
 -e git+https://github.com/edx/js-test-tool.git@v0.1.5#egg=js_test_tool