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 convenient methods to ConfigQuery to get store_email and store_description configuration variables.

It defines some convenient methods in MailerFactory to send an email built from a e-mail template to the customer, the store managers, or everybody else.

It's now possible to change the customer email address in the Back Office.

A new "Password sent" page (route /password-sent) has been added to Front module, to inform the customer that a new password was sent to his email address.

According to a slow background refactoring process, the "message" part  of the back office uses now the standard fields renderers (render_form_field, etc...)