Allow turning off Video CDN in Studio.

BLD-1207
Annotator Tools: Fix major logic bug in code
Add Optimizely widget
Merge pull request #4760 from edx/oleg/turn-off-cdn-rc

Allow turning off Video CDN in Studio.
Merge pull request #4764 from edx/diana/show-cert-without-end-date

Show Certificates Before End
Merge pull request #4763 from edx/lms/add-optimizely

Lms/add optimizely
Merge pull request #4762 from edx/annotations-js-fix

Annotator Tools: Fix major logic bug in code (RC)
Reenable user partition in Advanced Settings.
Merge pull request #4775 from edx/alex/fix-reenable-user-partitions

Reenable user partition in Advanced Settings.
Update xblock-mentoring.

Update reference to the xblock-mentoring repository.

A bug where answers containing the string "??" would get mangled by jQuery has been fixed in xblock-mentoring.
LMS-11177 Add Split to configuration settings.
LMS-11179 Fix Assets with Split
LMS-11019 Add feature flag to create Split Course
LMS-11168 Support for removing versions and branch in Split, Mixed, and SQL.
Make default_store thread-safe.
Fix get_courses.
Make Split Course work with new Publishing UI.
Address code coverage and quality.
get_items API: have qualifiers be a separate parameter instead of assuming kwargs.
Update redirect and wording for prof ed

- Changes registraiton button text for professional ed courses
- Professional ed courses do not see "choose your track" page; go straight to verification flow
Adapt flow for new 'professional' mode.

ECOM-41