[IMP] mass_mailing: add responsible in mailing view

Otherwise it is not possible to change it in order to re-assign the mailing.
Also update the field label in order to better match its intended use.

This commit is linked to task ID 1907994 and PR #28533.

addons/mass_mailing/models/mass_mailing.py

@@ -470,7 +470,7 @@ class MassMailing(models.Model):
     state = fields.Selection([('draft', 'Draft'), ('in_queue', 'In Queue'), ('sending', 'Sending'), ('done', 'Sent')],
         string='Status', required=True, copy=False, default='draft', group_expand='_group_expand_states')
     color = fields.Integer(string='Color Index')
-    user_id = fields.Many2one('res.users', string='Mailing Manager', default=lambda self: self.env.user)
+    user_id = fields.Many2one('res.users', string='Responsible', default=lambda self: self.env.user)
     # mailing options
     reply_to_mode = fields.Selection(
         [('thread', 'Recipient Followers'), ('email', 'Specified Email Address')], string='Reply-To Mode', required=True)

addons/mass_mailing/views/mass_mailing_views.xml

@@ -618,6 +618,7 @@
                             <page string="Options">
                                 <group>
                                     <group string="Mailing">
+                                        <field name="user_id"/>
                                         <field name="email_from"/>
                                         <label for="reply_to"/>
                                         <div>