Fix university office/amenity collision (#1607)

Fixes #1606
Unblocks ZeLonewolf/openstreetmap-americana#1022

This PR is a bugfix which ensures that `amenity=university` is encoded as `class=college` rather than `class=office`.  It's required because of a subclass collision for `university`, which exists under both class values.

layers/poi/class.sql

@@ -33,6 +33,8 @@ CREATE OR REPLACE FUNCTION poi_class(subclass text, mapping_key text)
     RETURNS text AS
 $$
 SELECT CASE
+           -- Special case subclass collision between office=university and amenity=university
+           WHEN mapping_key = 'amenity' AND subclass = 'university' THEN 'college'
            %%FIELD_MAPPING: class %%
            ELSE subclass
            END;