celery-haystack keeps Celery and database busy

If Celery is available, the haystack index is updated asynchronously after virtually every request, causing a lot of celery tasks and collection of a lot of results.

Now that we decided to make celery non-optional, we should probably decouple this from the request and just let the index update once every minute or so. (the table collected 1.2 million rows in the course of 24 days, even updating the index every 10 seconds would only sum up to ~340,000 in that period).

In addition, or alternatively, we could ignore the result of these jobs.