Added average rating field to movie

- Average rating will be re-calculated when a user creates or updates
the rating instead of calculates it when a movie is displayed.
- Fixed has_many relations on Movie and User model.
- Replaced 'radio_button_tag' for 'radio_button' (requested by
jfgomez86 on
https://github.com/jfgomez86/movietracker/pull/27#discussion_r2518256)
Merge pull request #29 from EdgarOrtegaRamirez/master

Added average rating field to Movie and Fix has_many relations