[FIX] mail: mail_server_id

When a specific SMTP server is set on a mail template, it is never used
when the mail is sent. There are two reasons:
- `mail_server_id` is a read-only field set by an onchange. However, a
  read-only field is not sent to the server, so the information is lost.
- The information is stored in the context of the object `Mail`, but it
  is never used. Moreover, when the method `message_post` is called, the
  `Mail` object is not used to post the message, so the information is
  lost again.

The fix includes two parts:
- the `mail_server_id` field is not read-only anymore. Actually, there
  is no reason to make it read-only.
- the information is stored directly in the mail values and used at the
  appropriate moment.

Fixes #6554 (from v9.0)
opw-669958

addons/mail/models/mail_message.py

@@ -110,7 +110,7 @@ class Message(models.Model):
         help='Answers do not go in the original document discussion thread. This has an impact on the generated message-id.')
     message_id = fields.Char('Message-Id', help='Message unique identifier', select=1, readonly=1, copy=False)
     reply_to = fields.Char('Reply-To', help='Reply email address. Setting the reply_to bypasses the automatic thread creation.')
-    mail_server_id = fields.Many2one('ir.mail_server', 'Outgoing mail server', readonly=1)
+    mail_server_id = fields.Many2one('ir.mail_server', 'Outgoing mail server')
 
     @api.multi
     def _get_needaction(self):

addons/mail/models/res_partner.py

@@ -93,6 +93,7 @@ class Partner(models.Model):
 
         mail_values = {
             'mail_message_id': message.id,
+            'mail_server_id': message.mail_server_id.id,
             'auto_delete': self._context.get('mail_auto_delete', True),
             'references': references,
         }

addons/mail/wizard/mail_compose_message.py

@@ -216,7 +216,7 @@ class MailComposer(models.TransientModel):
             if wizard.template_id:
                 # template user_signature is added when generating body_html
                 # mass mailing: use template auto_delete value -> note, for emails mass mailing only
-                Mail = Mail.with_context(mail_notify_user_signature=False, mail_server_id=wizard.template_id.mail_server_id.id)
+                Mail = Mail.with_context(mail_notify_user_signature=False)
                 ActiveModel = ActiveModel.with_context(mail_notify_user_signature=False, mail_auto_delete=wizard.template_id.auto_delete)
             if not hasattr(ActiveModel, 'message_post'):
                 ActiveModel = self.env['mail.thread'].with_context(thread_model=wizard.model)
@@ -282,6 +282,7 @@ class MailComposer(models.TransientModel):
                 'email_from': self.email_from,
                 'record_name': self.record_name,
                 'no_auto_thread': self.no_auto_thread,
+                'mail_server_id': self.mail_server_id.id,
             }
             # mass mailing: rendering override wizard static values
             if mass_mail_mode and self.model: