Update ReactJS

[xMartin]

For the widgets like "upcoming events" or "discussion" we're using ReactJS. It's a pretty old version and although there is no urgent need to update I think it would be a good idea to use a more recent one so official documentation and tutorials can be used when working on the site.

It's actually a quite interesting task for anyone with some ReactJS experience because we're not using any build tool or JSX so you can see how to develop simple frontend code with pure JavaScript/ReactJS.