Implement in-place address creation in shipping form

Fix #307 (closed) - Here is the new workflow:

1. When no addresses exist, disable drop-down menus and provide links to create new address.

1. Clicking one of the links opens a modal.

1. If the address is created successfully, the modal closes and the new address is automatically selected in the drop-down. The drop-downs are now enabled and the user can create another address if needed.