[IMP] mail: hide Unsubscribe button in channels using group-based subscription

Group-based channels automatically add people belonging to user groups to the
channel. If a member leaves the channel he will be automatically added back
by the channel automatic subscription. This generates unnecessary changes and
messages in the channel.

We now prevent people from leaving group-based channels. All group members
have to be channel members. Unsubscribe buttons are therefore removed for
those channels.