Merge pull request #650 from mozilla/feature/spawn-chat-messages

src/assets/spawn_message-hover.png

src/assets/spawn_message.png

src/assets/stylesheets/presence-log.scss

src/assets/stylesheets/ui-root.scss

src/hub.html

src/hub.js

src/react-components/chat-message.js

src/react-components/presence-log.js

src/react-components/ui-root.js

src/scene-entry-manager.js

src/systems/userinput/bindings/keyboard-mouse-user.js

src/systems/userinput/paths.js

src/utils/serialize-element.js