Creating a new Organization based preference model.

This is for ECOM-682, a sub task of ECOM-525. Creating a new Organization based Preference model, mimicking the design of the Course based Preference model (UserCourseTag). 

Added test factory, and bolstered unit tests across all preference models.

Migration includes a tweak to the CourseKeyField for UserCourseTag. This should not have any impact on the DB migration itself, as the CourseKeyField extends a VarChar Field.

@wedaly @dianakhuang @rlucioni @feanil @shnayder @mikigoyal