Refactor review scores using value objects

**Refactor review scores using value objects:**
* Extract parent class for a generic review score
* Extract classes for media and user review scores
* Add helper methods to `UserReview`, `MediaReview` and `Event`
* Heavily refactor reviews helper to use review scores
* Update views to use refactored helper methods
* Slightly refactor CSS for review scores

http://blog.codeclimate.com/blog/2012/10/17/7-ways-to-decompose-fat-activerecord-models/#value-objects