Add dark invert class for images

[jorg-vr]

This pull request adds a css class that inverts images in dark mode, as an extra tool for users creating exercises.

I opted not to implement the requested data-large-dark atribute style css as that would add more rather complex css, while the only benefit is less duplication in the exercise description.

• Documentation update can be found at Add invert dark to docs dodona-edu.github.io#435

Closes #3175