feat(remote_mute): Add analytics event

JitsiConferenceEventManager.js

 
     chatRoom.addListener(XMPPEvents.AUDIO_MUTED_BY_FOCUS,
         value => {
+            Statistics.analytics.sendEvent('conference.remotelyMuted');
+
             // set isMutedByFocus when setAudioMute Promise ends
             conference.rtc.setAudioMute(value).then(
                 () => {