Implement conference broadcast feature.

It allows the organizer to choose whether participant are speakers or
listener in a conference.
For scheduled conferences, if a participant is not in the list of
allowed participants, he/she will have its role set to listener (i.e
audio stream will be sendonly from the server side and recvonly on the
client side).

Non scheduled conferences will have the organizer and participants to be
speakers.

The dynamic change of a participant role is not supported yet once the
conference is started but it may be done prior to it by updating the
conference informations