More flexible, uses the word "assert", method instead of function

This commit updates lms/djangoapps/courseware.

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]

with code to translate between addressing schemes based on app
and persistence layer addressing scheme specification.

STUD-1206

STUD-1006

Also cleans up test_view_authentication to use user factories for
various user classes

This centralizes the logic for group membership, and means that we
don't need to make queries to find out whether the legacy groups names exist.

Modified navigation tests to use MixedModulestore
Updated factories to find editable modulestore

Updated test_submitting_problems

Updated test_tabs.py

Updated test_view_authentication

Updated test_views

Updated courseware/tests/tests.py

Updated test_masquerade

Updated test_module_render

Pylint fixes

Updated video and word cloud tests

Updated course wiki tests

Updated license and open_ended tests.
One open_ended test still failing due to Mako initialization issues

Updated staticbook

Updated django_comment_client tests

Updated instructor tests

Updated instructor task tests

Updated external_auth tests

Updated course_groups

In the middle of addressing pull request comments. This is a safety commit in case I have to revert some changes I'm about to make.

Conflicts:
	lms/djangoapps/courseware/tests/test_navigation.py
	lms/djangoapps/courseware/tests/test_view_authentication.py
	lms/djangoapps/courseware/tests/tests.py