when a channel is update propagate the channel to everybody (#6677)

webapp/utils/constants.jsx

@@ -222,6 +222,7 @@ export const SocketEvents = {
     POST_DELETED: 'post_deleted',
     CHANNEL_CREATED: 'channel_created',
     CHANNEL_DELETED: 'channel_deleted',
+    CHANNEL_UPDATED: 'channel_updated',
     CHANNEL_VIEWED: 'channel_viewed',
     DIRECT_ADDED: 'direct_added',
     NEW_USER: 'new_user',