Demo quiz needed on the landing page

As we have learnt from ARSnova mobile, without a demo session people won't experience the full magic of the app.

I suggest to place a further big button "Demo Quiz" beneath the action buttons "Mach mit!" and "Mach neu!". Depending on the current localisation, a demo quiz is loaded from the repo (see "Info" at the bottom). The user has to solve a CAPTCHA to go on. The name should be automatically incremented, e.g. "Demo Quiz 42" starting with 1, and the user is brought to the quiz management view. The current highest number of the demo quiz must be looked up in MongoDB. This number will tell us how many people have tested our app.