Kitsune uses facets to calculate community activity metrics. Facets have been replaced in ES v2 by aggregations. To switch to aggregations we need to update ES python libraries to newer versions. Meanwhile IT wants to update ES to v2 from v1 (bug 1356411) As a workaround we can use ES and do the aggregation in the DB. Because the calculation will be slower, we'll cache results for 15 minutes.