Advanced feedback 2.0

[Willem3141]

We need a refinement of the work done in #176 because many checks can return several distinct feedback strings. We need separate templates for all of them, like this:

            "feedback": {
                "passed": {
                    "template": "Correct!",
                    "description": "correct"
                },
                "too_many": {
                    "template": "Your answer had [[actual]] vertices; more than the [[expected]] expected",
                    "fields": ["actual", "expected"],
                    "description": "too many vertices"
                },
                "too_few": {
                    "template": "Your answer had [[actual]] vertices; few than the [[expected]] expected",
                    "fields": ["actual", "expected"],
                    "description": "too few vertices"
                }
            }

And then in the Python code for example:

        return {'correct': False,
                'feedback': 'too_few',
                'fields': { 'actual': ..... } }