Previously, CourseGraph needed to be kept up-to-date by
running `./manage.py dump_to_neo4j ...` manually or on a cron timer.
This introduces a CMS new setting: COURSEGRAPH_DUMP_COURSE_ON_PUBLISH.
When enabled, the CMS course_published signal handler will
asynchronously dump each individual course to CourseGraph when it
is published.
This follows a pattern established by other subsystems like
learning_sequences and special exam registration, both of which
fire off asynchronous post-processing tasks from the course-
publish handler.
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