implement welcome message and clear message in group chat messages

...ts/api/group-chat-msg/server/modifiers/addGroupChatMsg.js

.../api/group-chat-msg/server/modifiers/clearGroupChatMsg.js

...html5/imports/api/meetings/server/modifiers/addMeeting.js

...l5/imports/ui/components/chat/chat-dropdown/component.jsx

bigbluebutton-html5/imports/ui/components/chat/service.js