multi_livechat: subscribe channel members the same as it is done with regular channels

[trojikman]

Problem:
now to subscribe members in integrations like sync_telegram or sync_viber we need to edit OPERATOR_IDS parameter.
I think that a more convenient way is to add the possibility to choose the group that will be subscribed and will have access to channels.
And it would be convenient to be able to edit the channel settings

[trojikman]

depends on #288

[trojikman]

estimation: minimum 2 hours - expected 3 - maximum 5