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.
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