fix(JitsiConference) send mute sourceInfo before removing an unmuted track

Workarounds around issue #2037 to have other participants receive the mute event and therefore avoid frozen camera bug

JitsiConference.js

     // Now replace the stream at the lower levels
     return this._doReplaceTrack(oldTrackBelongsToConference ? oldTrack : null, newTrack)
         .then(() => {
+            if (oldTrackBelongsToConference && !oldTrack.isMuted() && !newTrack) {
+                oldTrack._sendMuteStatus(true);
+            }
             oldTrackBelongsToConference && this.onLocalTrackRemoved(oldTrack);
             newTrack && this._setupNewTrack(newTrack);