Make the activity page update with AJAX

The activity page looks the same as the job page. So it should behave
the same and not need manually refreshing to see the new stuff.

tests/app/main/views/test_jobs.py

@@ -338,6 +338,15 @@ def test_can_show_notifications(
         assert csv_response.get_data(as_text=True) == csv_content
         assert 'text/csv' in csv_response.headers['Content-Type']
 
+        json_response = client.get(url_for(
+            'main.get_notifications_as_json',
+            service_id=service_one['id'],
+            message_type=message_type,
+            status=status_argument
+        ))
+        json_content = json.loads(json_response.get_data(as_text=True))
+        assert json_content.keys() == {'counts', 'notifications'}
+
 
 def test_should_show_notifications_for_a_service_with_next_previous(
     app_,