Feature/123 make learning clearer

[Nitwix]

Wraps images from learning UI inside a card view to have rounded corners and a border that indicates with which element to interact.

The idea was that first the image to sort has a green border to indicate that you should interact with it. Then, once you start dragging it, the target categories have the green border, to indicate you should drag the image on one of the targets. If it is wrong, the border of the category turns red to indicate a mistake.

This could not be implemented yet because of a lack of time on my part. I'll add an issue so that we can implement this in a future sprint.

[Nitwix]

It seems there is a compilation error. Fix in progress