fix: sink ccx courses on original course published

[Ian2012]

Description: This PR sinks the CCX child courses when the master course is published.

Testing instructions:

1. Enable CCX courses
2. Create a CCX course
3. Publish the master course
4. Verify that the CCX child courses are sink

Author concerns

Although there is a signal receiver for the course_published signal in the CCX Django application, this application has been written for the LMS, so the signal is never connected. Refactoring it would require a major development effort that's out of scope for this project. Also, doing that refactoring will not be compatible with older edx-platform releases.

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[bmtcril]

LGTM!

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