Group functionality: group avatars, group updates (full support), group leaves

group functionality: 
• group avatar support 
• group update support (full comptibility iOS/android)
• group leave support
delete thread left out for now as I'm not sure the appropriate action on android to make an equivalent on iOS (does this mean we just remove the thread from the DB? or also send a leave group message and take some additional actions)? in any case I think it is a not going to involve more than a one-two liner here.