Semantic-ui-react: Update to React 16

[levithomason]

React 16 offers some updates that we should implement, however, we'll need to drop support for React 15 when doing so. Let's batch these changes into an upcoming major release:

• [x] #2746 Portal to use ReactDOM.createPortal (https://reactjs.org/docs/portals.html)
• [ ] Refactor and simplify Accordion to return an array of panels (https://reactjs.org/docs/react-component.html#render)
• [ ] Refactor and simplify Breadcrumb to return an array of sections/dividers (https://reactjs.org/docs/react-component.html#render)
• [ ] #2306 Fix ref issues using forwardRef (https://reactjs.org/docs/forwarding-refs.html)
• [x] #2842 Use PureComponent for Icon and Flag

@Semantic-Org/react-maintainers feel free to add to this list at will...

[layershifter]

Added Breadcrumb to list, there is quite unfriendly code.

[packetstracer]

Maybe you should add a Fix for unsafe lifecycle methods for asynchronous rendering.

In the project I'm working currently, since we activated the new React's Strict Mode, we are getting a lot of console warnings about using unsafe lifecycle methods y some of the components.

For example:

componentWillMount: Please update the following components to use componentDidMount instead: Dropdown

componentWillReceiveProps: Please update the following components to use static getDerivedStateFromProps instead: Dropdown, Menu, Tab

componentWillReceiveProps: Please update the following components to use static getDerivedStateFromProps instead: AccordionAccordion

componentWillReceiveProps: Please update the following components to use static getDerivedStateFromProps instead: Checkbox

[layershifter]

@packetstracer see #2732, feel free to contribute 👍