Various issues caused by pressing enter key immediately after entering a saved user's email

[dan-lovelace]

Environment: test mode
Platform: Ubuntu 16.04 - Firefox 59
Use cases: 2

Use case 1

1. Enable billingAddress property
2. Navigate to the checkout modal
3. Complete all address fields
4. Enter a saved email into the email field
5. Press the enter key before the verification code prompt is shown
6. Notice the fields are rendered on an empty background

Expected:
The system will gracefully handle the user pressing the enter key before the verification code prompt is rendered.

Screenshot: https://www.screencast.com/t/YgOejgCec6z

Use case 2

1. Disable billingAddress property
2. Navigate to the checkout modal
3. Complete all credit card fields
4. Enter a saved email into the email field
5. Press the enter key before the verification code prompt is shown
6. Notice checkout is successful

Expected:
The system will not allow saved users to checkout without entering a verification code.