remove chat collection and fix group chat messages subscription

bigbluebutton-html5/imports/api/chat/index.js

bigbluebutton-html5/imports/api/chat/server/eventHandlers.js

...ton-html5/imports/api/chat/server/handlers/chatHistory.js

...ton-html5/imports/api/chat/server/handlers/chatMessage.js

...mports/api/chat/server/handlers/chatPublicHistoryClear.js

bigbluebutton-html5/imports/api/chat/server/index.js

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

...imports/api/chat/server/methods/clearPublicChatHistory.js

...ebutton-html5/imports/api/chat/server/methods/sendChat.js

...button-html5/imports/api/chat/server/modifiers/addChat.js

...ton-html5/imports/api/chat/server/modifiers/clearChats.js

...orts/api/chat/server/modifiers/clearUserSystemMessages.js

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

...ton-html5/imports/api/group-chat-msg/server/publishers.js

...imports/api/group-chat/server/modifiers/clearGroupChat.js

...mports/api/group-chat/server/modifiers/removeGroupChat.js

...l5/imports/api/meetings/server/modifiers/removeMeeting.js

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

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