Visually represent review scores

**Visually Display Review Scores:**
* Display review scores instead of placeholder images
* Extract helpers for review score `div`s
* Extract helpers to for captioned (averaged) review scores
* Apply CSS class to review score based on value
* Update CSS to display colored square with review score
* Add default (gray) color to CSS for review scores

**Refactorings:**
* Extract CSS for review scores to separate file
* Rename `review_statistics_helper` to `reviews_helper`
* Rename score formatting helpers for clarity (and update views)
* Change CSS selectors for `review-image` to `review-score`

https://trello.com/c/AJntujhL