Added the following fields to CourseOverview:
announcement
catalog_visibility
course_video_url
effort
short_description
Now requires CourseOverview models for each course in the system:
Introduced a CourseOverviewGeneratedHistory model to keep track of
when the CourseOverview table was seeded. The seeding can be
done preemptively by calling the generate_course_overview management
command. Otherwise, it is lazily computed, when needed.
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