Take advantage of new audit_events index in framework stats
Take advantage of new audit_events index in framework stats

When joining suppliers with audit events there's no need to have
a object_type filter, since framework interest events are always
related to a supplier object. But adding additional object_type
filter allows the DB to take advantage of the new multicolumn
(object_type, object_id) index.

alphagov/digitalmarketplace-api,alphagov/digitalmarketplace-api,alphagov/digitalmarketplace-api