Added average rating field to Movie and Fix has_many relations

- Average rating will be re-calculated when a user creates or updates the rating instead of calculates it whenever a movie is displayed.
- Added index to average_rating column for fastest searches.
- Removed average_rating method on Movie model
- Fixed has_many relations on Movie and User model.
- Replaced 'radio_button_tag' for 'radio_button' (https://github.com/jfgomez86/movietracker/pull/27#discussion_r2518256)