Add rating star CSS thingy
Add Opinion model
Add uniqueness constraint to Opinion
Add opinion report to Thing show page

- [completes #66720648]
Make a couple styling changes and add seeds
Add average rating to Thing page

- [Completes #66724520]
Add Opinion form and OpinionsController#create
Add validations for rating and comment on Opinion

[Completes #66727664]
Add ability to update opinion

[Completes #66727664]
Refactor a bit
Correct styling on star rating
Add Thing#average_rating
Add ThingsController#index.json

- Also add failing spec for ThingsController#show.json (to be passed by code in next commit)