[FIX] mail: mention a channel from a channel
[FIX] mail: mention a channel from a channel

Before this commit, when mentioning a channel from a channel,
it crashed with following error:

```
TypeError: unhashable type: 'list'
```

This commit fixes the issue by providing a list of channel ids,
instead of a list of `[4, channelID]`, when mentioning channels.

closes odoo/odoo#36359

Signed-off-by: Alexandre Kühn (aku) <22220c18caede9806cb42c8e2b41136d711673bf@odoo.com>
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo