Fix missing import on notification stats endpoint

The response import was missing, which broke things.

There was no test to catch this.

This commit adds a rudimentary test to make sure that the endpoint
doesn’t 500, at the very least.

app/notifications_statistics/rest.py

@@ -1,6 +1,7 @@
 from flask import (
     Blueprint,
     jsonify,
+    request
 )
 
 from app.dao.notifications_dao import (