CourseFavourite           -- which user accessed which course when, only displayed to user for convenience;
    user         UserId
    course       CourseId
    reason       FavouriteReason
    lastVisit    UTCTime
    UniqueCourseFavourite user course
CourseNoFavourite
    user         UserId
    course       CourseId
    UniqueCourseNoFavourite user course