FIX #5799 sendmail : use model and signature in raw body

modules/sendmail/js/functions.js

modules/sendmail/lang/en.php

modules/sendmail/lang/fr.php

modules/sendmail/mail_form.php

modules/sendmail/sendmail_to_contact_ajax.php

modules/templates/templates_ajax_content_for_mails.php

modules/templates/xml/IVS/validation_rules.xml