Do not insert contact mechanism between recipients hidden fields

Since e356d97a0f59 there are two recipients hidden fields in the email
template view.

Closes #12981
(grafted from 37263c3bb8110b0cbf470ace7d2cd779defb66b4)

--HG--
branch : 6.0

modules/party/view/email_template_form.xml

@@ -2,7 +2,7 @@
 <!-- This file is part of Tryton.  The COPYRIGHT file at the top level of
 this repository contains the full copyright notices and license terms. -->
 <data>
-    <xpath expr="//field[@name='recipients_hidden']" position="after">
+    <xpath expr="//field[@name='subject']" position="before">
         <label name="contact_mechanism"/>
         <field name="contact_mechanism" colspan="3"/>
     </xpath>