swarm: follow user preference for hosting conferences

This avoid mobile phone to host conferences for others unless
explicitly authorized by the user.
Clients can show it via the "hostConference" conversation's preference.

Documentation: https://docs.jami.net/technical/swarm.html#call-in-swarm
GitLab: #312
Change-Id: I9bbd3e394cd1b3bcfd4a3120d9023a5ed686303d