Display and change of an anonymous user's email address in the checkout

[SGrueber]

Actual Behavior If an anonymous user starts the checkout he can enters an email address.

1. The email address is not shown on any checkout page
2. If the user navigates back to the checkout address page and enters another invoice address address the email address gets lost and the user is able to place the order without email address.
3. The email address cannot be changed by the user during checkout.

Expected Behavior

1. The email address of an anonymous user should be always be displayed as part of the invoice address, i.e. in the invoice address widget of the checkout address page, the checkout right panel and the invoice address on review/receipt/order detail page

2. The anonymous user should not be able to enter a new invoice address, he can only change his invoice address. He can enter a new shipping address if his invoice and shipping addresses are equal.

3. For anonymous users the email address should be added to the invoice address form on the checkout address page.

AB#80370