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
Nicolas Martinelli
8b854ab660