Show cats

Users can see cats by id.
This doesn't actually have to be an instance variable. You can just use:

```ruby
cat = Cat.create(name: ....)

visit cat_path(cat)

expect(page).to have_content(cat.name)
expect(page).to have_css("img[src=\"#{cat.url}\"]")
```

The only reason we use instance variables in our controllers is when we need to pass data into the view.