Export to arsnova click

This MR adds a button to the showcase scenario for exporting the current question in a format that's importable to ARSnova.click. When importing to click, the user is asked to give a name to the session and is then directly routet to the lobby

As of now, the lecturer has no possibility to adjust settings for the click session (e.g.: Timer, whether reading confirmation is required and futhrer more). It just exports so that the user doesn't lose time. I'd be possible to either add another view so that the lecturer can specify attributes like timer, or just use the default values arsnova.click is using when nothing's specified. Suggestions?