conference: a conference can now be controlled from authorized accounts

Implement application+confOrder to allow a remote peer to control the
conference's layout. The authorized peer can send a json likes:
{
    "layout": 1,
    "activeParticipant: "uri"
}
For now authorized peers are local accounts. This also change the conference
layout by sending "isMaster" for each participants to allow the peer to know
if we are the master when we are not the host.

Change-Id: Iba8a55453421e3f173b43e773d1b43502efec747
Gitlab: #241