Use Google Analytics Enhanced ecommerce.

Hi,

I have implemented the Google Analytics Enhanced ecommerce based on discussion in #5547.
So far I have done:
1) The purchase part, which ensures that this pull har feature parity with the old Google Ecommerce implementation
2) The checkout part. This will report in Analytics which step the user has proceeded to. 

There are several other parts which also should be implemented, such as "Impression data", "Click Data" and "Refund". However, I wanted to share this pull now to see if the way I have implemented it is the way that you would like it to be implemented. My approach is based on the discussion in issue #5547.

I see this pull as a discussion starter on the implementation details and would love your feedback on it.

PS: I know that you want tests, but I'm very unsure how to test this feature? 


Just to be 100% sure, this is what you would like to see:
`<%= render partial: 'spree/shared/google_checkout.js', locals: { order: @order, step_number: checkout_states.index(@order.state) + 1 } %>