Add tracking to checkout flow

[willopez]

Resolves #379
Impact: minor
Type: feature

Summary

Add tracking to the checkout flow, see #379 for event details

The following events will be published to Segment:

• Checkout Step Viewed | User viewed a checkout step
• Checkout Step Completed | User completed a checkout step
• Payment Info Entered | User added payment information

Testing

1. Add a couple of items to the cart
2. Click on checkout
3. Once on the checkout page, verify that "Checkout Started" and "Checkout Step Viewed" events are published
4. Completed the first step
5. Verify that a "Checkout Step Completed" was published for step 1, and a "Checkout Step Viewed" for step 2
6. Complete step 2, Verify that a "Checkout Step Completed" was published for step 2, and a "Checkout Step Viewed" for step 3
7. Complete step 3, Verify that a "Payment Info Entered" was published for step 3, and a "Checkout Step Viewed" for step 4
8. Complete step 4, Verify that a "Checkout Step Completed" was published for step 4

[mikemurray]

👍

[mikemurray]

Approved, but waiting for a minor lint issue to be resolved