add option for disabling join/leave sounds (#8596)

* add option for disabling join/leave sounds

* document disableJoinLeaveSounds and add it to whitelist

config.js

     // Decides whether the start/stop recording audio notifications should play on record.
     // disableRecordAudioNotification: false,
 
+    // Disables the sounds that play when other participants join or leave the
+    // conference (if set to true, these sounds will not be played).
+    // disableJoinLeaveSounds: false,
+
     // Information for the chrome extension banner
     // chromeExtensionBanner: {
     //     // The chrome extension to be installed address

react/features/base/config/configWhitelist.js

     'disableH264',
     'disableHPF',
     'disableInviteFunctions',
+    'disableJoinLeaveSounds',
     'disableLocalVideoFlip',
     'disableNS',
     'disableProfile',

react/features/base/participants/middleware.js

  */
 function _maybePlaySounds({ getState, dispatch }, action) {
     const state = getState();
-    const { startAudioMuted } = state['features/base/config'];
+    const { startAudioMuted, disableJoinLeaveSounds } = state['features/base/config'];
+
+    // If we have join/leave sounds disabled, don't play anything.
+    if (disableJoinLeaveSounds) {
+        return;
+    }
 
     // We're not playing sounds for local participant
     // nor when the user is joining past the "startAudioMuted" limit.