Adding an image-with-shadow CSS class

[tobyhodges]

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.)

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