Rework send invitation API: do not give chat room params but a boolean stating...

Rework send invitation API: do not give chat room params but a boolean stating whether the chat room should be encrypted or not

Add ICS cancel/update capability.
-> Update: participants already invited receive an ICS with the same UID and the sequence incremented by one when the information up to date
-> Cancel: participants that are no longer part of the conference, receive such ICS

In order to globally cancel a conference, an admin can send an INVITE to the conference with an empty list of participants

time SDP field (t=) is used to know whether the INVITE isaiming at joining a conference or update it:
- if t=0 0, the client wishes to join a conference
- if t!=0 0, the client wishes to update the informations of a conference (subject, start time, duration, participants)