conference: add a rendez-vous mode

This patch aims to be a first iteration on a new conference mode. Actually,
creating a conference for many participants is pretty boring and the UX is
bad because for any conference, we need to call any participant and add them.

This new mode transform an account into a meeting account. This means whenever
this account receives a call (from an authorized peer) the call is answered
and added to the current conference. So, an account can easily host a meeting.

Gitlab: #269
Change-Id: I5ece585f8eb46f5cf6a5f9e3dc56829949cc7a81