Adding an image-with-shadow CSS class

In the DC Image Processing with Python lesson, we made use of a class called `image-with-shadow` in the old template to make sure that the boundaries of some images were visually distinct from the background of the lesson page. ([See an example of this in use](https://github.com/datacarpentry/image-processing/blob/c5349b5d49f0b5d093cc551680c5be9bcc998c47/episodes/04-drawing.md?plain=1#L125).)

Although not urgent, it would be very helpful to have a similar class available in the new version of the lesson post-Workbench transition.