Add ping-pong message to html5 client

bigbluebutton-html5/imports/api/ping-pong/index.js

bigbluebutton-html5/imports/api/ping-pong/server/index.js

bigbluebutton-html5/imports/api/ping-pong/server/methods.js

...button-html5/imports/api/ping-pong/server/methods/ping.js

...uebutton-html5/imports/api/ping-pong/server/publishers.js

...l5/imports/api/users/server/handlers/validateAuthToken.js

...ton-html5/imports/api/users/server/methods/userLeaving.js

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

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

bigbluebutton-html5/imports/startup/server/index.js

bigbluebutton-html5/imports/ui/components/app/component.jsx

...utton-html5/imports/ui/components/ping-pong/container.jsx

...n-html5/imports/ui/components/subscriptions/component.jsx

bigbluebutton-html5/server/main.js