Refactor MoviePreference to have a property for the related Movie

[zenovy]

Type 'Movie' should encapsulate data about the movie, whereas type 'MoviePreference' should encapsulate data about the user's relationship to the movie. However, the latter contains a lot of data about the movie itself. We should remove the properties related to the movie from MoviePreference and instead access that data through a property on MoviePreference of type Movie.