:construction: Integrate merchant gem

Mustache is just too painful because you can't see half of the errors.

The almost-empty view in app/views is needed for rspec to run at all (makes me a bit sad). With some logic in the view finding code we should be able to make sure that it never gets displayed to visitors.

This brings up the question: do we test client-specific views, and how do we do it in a maintenable question? Definitely something to keep in mind as we proceed.

I'm keeping Product inside the catalog for now until I add more classes and I figure out how I want to lay them out.

I am a bit worried about the amount of setup code for the catalog, though. Any ideas in that area would be greatly appreciated (how to rearrange the whole object graph).

## Todo

- [ ] Remove mustache view objects
Wrap it into a builder object :)

Something like :

```ruby
class ProductCatalogBuilder
  def initialize(slug:, context:)
    @slug = slug
    @context = context
  end

  def self.for_client(slug:, context:)
    new(slug: slug, context: context).catalog
  end

  def catalog
    ProductCatalog.new(context, merchant)
  end

  def merchant
    CnC::Merchant.build endpoint: Roundular.products_api_endpoint,
      client_token: Roundular.client_tokens[@slug]
  end
end
```