Fix youtube keeps playing audio when user left the meeting

bigbluebutton-html5/imports/api/users/server/methods.js

...html5/imports/api/users/server/methods/userLeftMeeting.js

...utton-html5/imports/api/users/server/modifiers/addUser.js

bigbluebutton-html5/imports/startup/client/base.jsx

...rts/ui/components/nav-bar/settings-dropdown/component.jsx