Changed play and next links to buttons

[jannispaul]

The play and next elements should be buttons instead of links.

[pganssle]

Sorry, I'm not the world's best front-end person, what is the advantage of these being buttons instead of links?

Also, can you move the commit from #45 into this one, so that if we merge them they can be merged together?

[jannispaul]

No worries :) It bugged me that I couldn't select the play and next links with my keyboard. That's because they dont have href-attributes. But instead of adding hrefs I suggest to use buttons, since they are the semantically correct element to use here. From the MDN documentation:

„Anchor elements are often abused as fake buttons by setting their href to # [...] Use a button instead. In general, you should only use a hyperlink for navigation to a real URL." https://developer.mozilla.org/en-US/docs/Web/HTML/Element/a#onclick_events

I tried to merge the commits but don't know how to do it. Sorry!