Do not add any attribute such as content, label or rtpmap if a stream is

inactive
If the offerer doesn't put down a content of the main stream when the
call is in a conference, then append the main stream instead of
searching for the best one in the offer as this search might hit a
participant stream
Improve layout deduction logic: if the client SDP has no stream with main
content but at least one recvonly stream without mlabel, then guess the
client layout to be Grid
Rework thumbnail stream index search by looking only at the client SDP
Add function that verify conference info deletion on the server