[FIX] email_template: default model for save as new template feature

When sending a message with the "Compose new message" button on the right of the user menu, in the top bar, if you tried to save the message as template, you had a traceback because model field of email.template is mandatory, but was set to True because there is no model in such a case.

As there is no any relevant model in such a place, and that the field is mandatory, mail.message is pretty convenient as the default value.

addons/email_template/wizard/mail_compose_message.py

@@ -131,7 +131,7 @@ class mail_compose_message(osv.TransientModel):
         email_template = self.pool.get('email.template')
         ir_model_pool = self.pool.get('ir.model')
         for record in self.browse(cr, uid, ids, context=context):
-            model_ids = ir_model_pool.search(cr, uid, [('model', '=', record.model)], context=context)
+            model_ids = ir_model_pool.search(cr, uid, [('model', '=', record.model or 'mail.message')], context=context)
             model_id = model_ids and model_ids[0] or False
             model_name = ''
             if model_id:

addons/email_template/wizard/mail_compose_message_view.xml

@@ -29,7 +29,7 @@
                         <div>Use template
                             <!--FIX: To avoid css issue of many2one field in footer temporary used oe_form (BUG:1152464)-->
                             <field name="template_id" nolabel="1" class='oe_inline'
-                                on_change="onchange_template_id(template_id, composition_mode, model, res_id, context)" domain="[('model_id.model','=',model)]"
+                                on_change="onchange_template_id(template_id, composition_mode, model, res_id, context)" domain="[('model_id.model','=',model or 'mail.message')]"
                                 context="{'default_model': model, 'default_body_html': body, 'default_subject': subject}"/>
                         </div>
                         <button icon="/email_template/static/src/img/email_template_save.png"