virtualized chat tweaks for rendering and performance

bigbluebutton-html5/imports/ui/components/chat/container.jsx

...ml5/imports/ui/components/chat/message-list/component.jsx

...ponents/chat/message-list/message-list-item/component.jsx

...ponents/chat/message-list/message-list-item/container.jsx

...omponents/chat/message-list/message-list-item/styles.scss

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