Add all exams button

Implements #71 (closed). I need an additional http request for the category metadata as the client needs the slug for the link - the exam metadata only includes category. Although it wouldn't be too hard to adjust the category page to work with both slug and name it would mean that multiple urls would represent the same category. I considered adding it to the server response, but decided against it as a category metadata is rather lightweight and the request doesn't interfere with the rendering.