Render POIs for large universities at low zoom (#1479)

This PR adapts the code used in #1457 to allow POIs to render up to z10 for very large features. I set the threshold at a very conservative value of 10% of a tile. Thus, it will render if an area POI covers at least 10% of a tile at a given zoom. Only for 'university' and  'college'.

Additionally, this consolidates a double UNION ALL on the same table with a WHERE/CASE combination, which is simpler.

layers/poi/mapping.yaml

@@ -327,6 +327,8 @@ def_poi_fields: &poi_fields
     type: id
   - name: geometry
     type: geometry
+  - name: area
+    type: area
   - name: name
     key: name
     type: string