user see reviews for cat and add new review

Doesn't include user info yet, so we'll have to merge master into our branch, but let us know if there are other changes we should make.
There is a shorthand for this:

```ruby
def create
  @cat = Cat.find(params[:id])
  @review = @cat.reviews.build(review_params)

  if @cat.save
    # ...
  else
    # ...
  end
end
```