Allow to associate a URL to a room, for example a link to a video
conference room. Tracks assigned to this room will automatically inherit
the link. The link for a given track can be overridden using the "url"
command.
This allows organizers to provide video links for each room, while still
enabling teams to link to whatever they end up really using.
Change-Id: I633eb83c579e4093ae769bcd053e4d2ec2fe4bc3
5538127768
