[FIX] mail: auto subscription to channel

1. Install mail and hr
2. Create a new mail channel:
   name: test
   Who can follow: selected group of user
   Authorized group: Employee / Manager
   Auto subscription: Employee / Manager
3. Edit the Demo user, and add him to group Employee Managers

The Demo user is not subscribed to the channel (only Admin is)

In this case, the web client sends a `sel_groups_` key, which is not
handled by the `write` method.

Fixes #20897
opw-782301

addons/mail/models/res_users.py

@@ -2,6 +2,7 @@
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
 
 from odoo import _, api, exceptions, fields, models
+from odoo.addons.base.res.res_users import is_selection_groups
 
 
 class Users(models.Model):
@@ -54,11 +55,14 @@ class Users(models.Model):
     @api.multi
     def write(self, vals):
         write_res = super(Users, self).write(vals)
+        sel_groups = [vals[k] for k in vals if is_selection_groups(k) and vals[k]]
         if vals.get('groups_id'):
             # form: {'group_ids': [(3, 10), (3, 3), (4, 10), (4, 3)]} or {'group_ids': [(6, 0, [ids]}
             user_group_ids = [command[1] for command in vals['groups_id'] if command[0] == 4]
             user_group_ids += [id for command in vals['groups_id'] if command[0] == 6 for id in command[2]]
             self.env['mail.channel'].search([('group_ids', 'in', user_group_ids)])._subscribe_users()
+        elif sel_groups:
+            self.env['mail.channel'].search([('group_ids', 'in', sel_groups)])._subscribe_users()
         return write_res
 
     def _create_welcome_message(self):