Add loading animation

[pmuens]

Add loading animation when data gets loaded from the backend.

[xndrdev]

What about https://github.com/quickleft/react-loader ? Pure js, no external images, no external css. @pmuens

[pmuens]

Looks good. But maybe we can find a packages which plays nicely with Redux and React (otherwise we have to dispatch loading actions which might be a little bit tedious).

[xndrdev]

What if we just show an CSS animated Spinner instead of the message here: https://github.com/JustServerless/discuss/blob/master/client/src/app/js/components/posts/index.jsx#L94 ?

[pmuens]

Good idea!