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.

Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

- Added tests
- Updated model field specification
- Fixed issue of multiple event emission
- Updated admin page
- Added management command to manually generate course structures