[WIP] Show broadcast channels in their own, proper "Channel" chat

[Hocuri]

Part of #6884

Note that I removed the following guard:

        if !new_chat_contacts.contains(&ContactId::SELF) {
            warn!(
                context,
                "Received group avatar update for group chat {} we are not a member of.", chat.id
            );
        } else if !new_chat_contacts.contains(&from_id) {
            warn!(
                context,
                "Contact {from_id} attempts to modify group chat {} avatar without being a member.",
                chat.id,
            );
        } else [...]

i.e. with this change, non-members will be able to modify the avatar. Things were slightly easier this way, and I think that this is in line with non-members being able to modify the group name and memberlist (they need to know the Group-Chat-Id, anyway), but I can also change it back.