Introduce DialogContainer for reusing dialog code

src/react-components/create-object-dialog.js

src/react-components/create-room-dialog.js

src/react-components/dialog-container.js

src/react-components/help-dialog.js

src/react-components/home-root.js

src/react-components/hub-create-panel.js

src/react-components/info-dialog.js

src/react-components/invite-dialog.js

src/react-components/link-dialog.js

src/react-components/report-dialog.js

src/react-components/safari-dialog.js

src/react-components/slack-dialog.js

src/react-components/ui-root.js

src/react-components/updates-dialog.js

src/react-components/webvr-recommend-dialog.js