[scorecards] use max not non negative score for percentage penalty

The percentage section penalty on the council page was using the non
negative score to calculate the percentage penaltry rather than the max
score for the sections.

Fixes #597

scoring/views.py

@@ -373,7 +373,7 @@ def get_context_data(self, **kwargs):
             if data.get("has_negative_points", False):
                 if data["non_negative_max"] != 0:
                     data["negative_percent"] = (
-                        data["negative_points"] / data["non_negative_max"]
+                        data["negative_points"] / data["max_score"]
                     ) * -100
                 else:
                     data["only_negative"] = True