Add loading indicator

[cmichi]

I looked into #72 and here's the thing: Most of the waiting time with a blank page is spent when initially loading the bundle.js, which contains react and the app. Since we would like to show the loading indicator during that time we can't reuse the MDSpinner component, since it is only available after React is loaded up.

As a compromise I suggest a minimal loading screen. I have intentionally left out any text, since the translations are also not yet available during the loading phase.

The <div id="loading-screen></div> is automatically overwritten once the app's render() method is invoked by react.

[chatch]

perfect. looks good.

hey btw. have you seen the current build challenge: https://www.stellar.org/lumens/build ? you can get rewarded for contributions.

[cmichi]

Great, thanks for the pointer and your quick feedback :-)!