Use arguments rather than passing around a dict

This makes it harder to write code which will pass tests but fail in
real life.

app/notify_client/performance_dashboard_api_client.py

@@ -3,8 +3,19 @@ from app.notify_client import NotifyAdminAPIClient
 
 class PerformanceDashboardAPIClient(NotifyAdminAPIClient):
 
-    def get_performance_dashboard_stats(self, params_dict=None):
-        return self.get("/performance-dashboard", params=params_dict)
+    def get_performance_dashboard_stats(
+        self,
+        *,
+        start_date,
+        end_date,
+    ):
+        return self.get(
+            '/performance-dashboard',
+            params={
+                'start_date': str(start_date),
+                'end_date': str(end_date),
+            }
+        )
 
 
 performance_dashboard_api_client = PerformanceDashboardAPIClient()