Card slider buttons refactoring to use Keen slider library

Original Text from Raphael:

I cannot reproduce the bug. This issue can only be a reminder. Big behaviour was that the "next" button made the cards move just a littel step (a fraction of a card width). That was strange.

We should probably refactor those buttons/carousels to use the Keen carousel (which is the lib we're already using in the Image Slider) instead of our own logic.

Addition from Moritz:

Currently, the swiping/scrolling logic inside of the CardSlider is a custom logic and it's not working very well. Instead, I'd love to replace it with KeenSlider, so we're using the same code/library to scroll images as we do with News/Events cards.