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

This commit updates common/lib/xmodule.

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]
Merge remote-tracking branches 'edx/opaque-keys-base-keys', 'edx/opaque-keys-cms', 'edx/opaque-keys-common-djangoapps', 'edx/opaque-keys-common-lib-xmodule', 'edx/opaque-keys-common-misc', 'edx/opaque-keys-django-fields', 'edx/opaque-keys-embargo-tests', 'edx/opaque-keys-library', 'edx/opaque-keys-lms-courseware', 'edx/opaque-keys-lms-djangoapps', 'edx/opaque-keys-lms-templates', 'edx/opaque-keys-mongoengine-fields', 'edx/opaque-keys-requirements' and 'edx/opaque-keys-roles-table' into opaque-keys
Revert StudentModule.module_state_key to field rather than computed property