Tracking campaigns on orders

[loevgaard]

See docs: https://mailchimp.com/developer/guides/marketing-api-conventions/#e-commerce

When a user clicks a link in an email from Mailchimp, Mailchimp will add mc_cid and mc_eid query parameters. The value in the mc_cid is the campaign is which we should send back to Mailchimp when a customer completes an order in the same session. This will allow the store to see the efficiency of campaigns sent in Mailchimp