Add support for cemetery tagged as grave_yard (#1175)

Fixes #1057

This PR adds `amenity=grave_yard` to the `landuse` layer.  A unification function was implemented which encodes all `class=grave_yard` as `class=cemetery` in the tiles, which adds these features for existing users of `class=cemetery` with no change.  The unification function can serve as a basis for any other tags that we might want to unify in the `landuse` layer.

Tile rendering for an `amenity=grave_yard`.  ([Location](https://www.openstreetmap.org/way/857383420))
![image](https://user-images.githubusercontent.com/3254090/129456504-187d307a-2a20-4b13-af0a-a2850503bb4f.png)

Tile rendering for a `landuse=cemetery`.  ([Location](https://www.openstreetmap.org/way/385779531))
![image](https://user-images.githubusercontent.com/3254090/129456654-494c0eb4-9785-467c-b1a0-44abb7f9d5e9.png)

layers/landuse/class.sql

@@ -0,0 +1,10 @@
+-- Unify class names that represent the same type of feature
+CREATE OR REPLACE FUNCTION landuse_unify(class text) RETURNS text LANGUAGE plpgsql
+AS
+$$
+BEGIN
+  RETURN CASE
+    WHEN class='grave_yard' THEN 'cemetery'
+    ELSE class END;
+END;
+$$;