Add third group visibility state

This branch adds a third group visibility state: each group is either invisible, visible, or shared with respect to each contact.

Invisible means that the contact doesn't see any sign that we subscribe to the group. Visible means that the contact can send us messages in the group, but we won't send the contact messages in the group. Shared means that the contact can send us messages in the group and we'll send the contact any shared messages in the group.

This is a preparatory step for #756 (closed). I'll put up another MR with the changes that close that ticket.