Set contact mechanism select before email template subject

Closes #13812
(grafted from 36866d8c04b4f9a8d09b6f8736f8cc851e85abdc)

--HG--
branch : 7.0

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='subject']" position="before">
+    <xpath expr="//label[@name='subject']" position="before">
         <label name="contact_mechanism"/>
         <field name="contact_mechanism" colspan="3"/>
     </xpath>