Account changed / created mail to the customer and minor enhancements

This PR add two emails sent to the customer, when its account is 1) changed or 2) created by an administrator.

It defines some convenient methods to send an email to the customer, the store managers, or everybody else.

It's now possible to change the customer email address in the Back Office.

According to a slow background refactoring process, the "message" part  of the back office uses now the standard fields renderers (render_form_field, etc...)