Round images of chapter manager

[kleinicke]

The images of the chapter manager should always be displayed as a circle. Currently, the images are shown as an oval, when the uploaded image is not a perfect square.
So before rendering, the image should be internally cropped to a square.

[janosh]

There's no clean on-the-fly solution. We'd have to use wrapper divs or change how we handle the data for our chapter pages in Contentful. If we fetched the images directly instead of through markdown, we could use Contentful's Images API which has cropping functionality.