Support markdown in feedback

Submit feedback layout is very barebones, even though a key feature of submit is the fact that it can give feedback on submitted work. Adding newlines is not possible (or maybe added in a recent update?), and there are no styling options.

Therefore, I think the feedback fields should be interpreted as markdown. This can be rendered client side to make the integration simpler. Toast-ui is a very good library for rendering (and editing) markdown.

This also allows for much better automatic feedback output.