Improve design of customer management

[odrotbohm]

The customer management should be improved:

• Create a dedicated package for all customer management related functionality
• Extract logic of UserAccount creation and customer creation into a transactional service (currently the logic in ShopController.registerNew(…) consists of two steps and is not transactional).
• Turn RegistrationForm into a domain type.
• Extract dedicated controller and expose functionality in a more idiomatic way.

[odrotbohm]

That has long been fixed.