This will allow v1 code of CourseOverview to delete entries. While this is not a good thing in general (and future versions will ignore entries with higher versions than they support), this is necessary to prevent errors with the existing code in a rollback situation. Otherwise, old code trying to delete CourseOverview entries will fail with a foreign key constraint violation.
Open edX -------- This is the root package for Open edX. The intent is that all importable code from Open edX will eventually live here, including the code in the lms, cms, and common directories. Note: for now the code is not structured like this, and hence legacy code will continue to live in a number of different packages. All new code should be created in this package, and the legacy code will be moved here gradually.