Implement Django admin action to stop running tasks

Based on https://github.com/geosolutions-it/nexus-geonode/issues/351

## Overview
Admin must be able to stop harvestable resources retrieval and ingestion.
In this moment the only option is either to use the session API from an API client, or kill the task abruptly and restore the harvester status manually inside the DB, We need a Django admin action to do it.