Enforce type of the XBlocks Fields when set

This updates the XBlock dependency to edX/XBlock@***
and add the newly introduced enforce_type methods
Add xmodule fields enforce_type tests
Make enforce_type an optional flag, to avoid conflicts
enforce-type: set the edx/XBlock#200 merge hash
Add my name to AUTHORS
get_items API: have qualifiers be a separate parameter instead of assuming kwargs.
Merge pull request #4736 from edx/usman/lms11205-embargomiddleware-invalidkeyerror

Catch InvalidKeyError in course_id_from_url().
Merge pull request #4720 from edx/split/add-and-fixes

Split/add and fixes
Move certificates_show_before_end into a new variable

Mark it as deprecated, but maintain backwards compatibility.

ECOM-11
Remove hardcoding of key syntax for base url
and /static and /jump_to_id link substitutions
LMS-11198
Allow turning off Video CDN in Studio.

BLD-1207
Replace create_xmodule with create_xblock
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
Refactor xml_importer.py for easier reading.
Remove post-publish step.
Add make_course_key to modulestores, so that external clients don't need to assume a CourseKey subtype.

Conflicts:
	common/lib/xmodule/xmodule/modulestore/xml_importer.py
Trying to get split to work
Import depth first to help w/ publishing
fixup - re-enable Asset checking.
Annotator Tools: Fix major logic bug in code
Move about overview creation to common superclass
Merge pull request #4754 from edx/diana/show-cert-without-end-date

Show Certificates Before End