FEDX-176

Move Split-specific tests to test_split_modulestore.py.
Derive remaining tests from ModuleStoreTestCase.
Remove unused persistent_factory.py.

Make split_mongo assert block identity uniqueness only over (block_type, block_id) tuples [LMS-11364]

Refactor all callers (often replacing with more appropriate fn)

Also, added for_branch_setting method to old mongo and removed some
branch verification tests

Conflicts:
	common/lib/xmodule/xmodule/modulestore/mongo/draft.py
	common/lib/xmodule/xmodule/modulestore/split_mongo/split_draft.py
	common/lib/xmodule/xmodule/modulestore/xml_importer.py

- Move Mongo tests to mixed ddt tests
- Check if `fields` is None before updating search targets
- Fix for double course version

LMS-2936

Also, a bunch of ancillary cleanups.

Conflicts:
	common/lib/xmodule/xmodule/modulestore/tests/test_publish.py

Conflicts:
	cms/djangoapps/contentstore/management/commands/migrate_to_split.py
	cms/djangoapps/contentstore/management/commands/tests/test_rollback_split_course.py
	common/lib/xmodule/xmodule/modulestore/__init__.py
	common/lib/xmodule/xmodule/modulestore/mixed.py
	common/lib/xmodule/xmodule/modulestore/mongo/draft.py
	common/lib/xmodule/xmodule/modulestore/split_migrator.py
	common/lib/xmodule/xmodule/modulestore/split_mongo/split.py
	common/lib/xmodule/xmodule/modulestore/tests/test_split_migrator.py
	common/lib/xmodule/xmodule/modulestore/tests/test_split_w_old_mongo.py

LMS-2950 move delete_course to Mongo.

Refactor get_parent_locations STUD-1663

[LMS-2757]

STUD-1592, STUD-1603, LMS-2642

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]

STUD-1359

and remove attempt to make it unique. Instead raise exception on dupe.
STUD-1359

[LMS-226]
[LMS-2013]

on new Locators to make it clear that they have a different syntax
than Location.course_id altho they represent the same info.

and fix found errors.

usage_id implies self-sufficiency. The block_id is relative to its structure.

use to instantiate loc_mapper
test loc_mapper injection into split modulestore