Display "--" when no reviews and refactor review scores

**Display "--" instead of "0" when there are no reviews:**
* Disallow 0 scores for User Reviews
* Display gray box with '--' for no reviews

**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

https://trello.com/c/UoofkvMS

http://blog.codeclimate.com/blog/2012/10/17/7-ways-to-decompose-fat-activerecord-models/#value-objects