Snap to fixed centroids of various countries

* Add healthmap_country.centroid_override for countries whose centroids are not on land.
* Use this field in QC stage 2 - if the location has country precision and its HealthMap country has a centroid_override point, replace the location point with the centroid_override point.
* Refactor QCManager for consistency and to move some logging into the QCManager.
Very minor, but you could use location.setGeom(centroid); here