Essay exercise type - make it possible to add model solution

[tompurker]

What would be needed:
That a model answer could be entered for essay exercise tasks. Model answer would be displayed to students after students submit their answer. It would be good if the model solution could be formatted (bold, italics, headings, etc.).

This sort of a thing will have to be implemented for the self-review feature. After this feature will be implemented for self-review feature, it will be implemented for essay type exercises as well.