conference: fix join between 2 conferences

Participants were detached from conf1 and attached to conf2 one by one, the last participant was detached but not removed from conference object because manager didn't consider the call as conference anymore because there was only 1 participant left.

Instead we detach and remove all participants from conf1 before add them to conf2.

Change-Id: Iac8c4d52d775397b94029fbdf90abf86103bfca5
Gitlab: #164