Add other river-like water area features to class=river (#1467)

Currently, river areas (`natural=water` + `water=river`) are rendered in the tiles as `class=river`.  However, canal, stream, ditch, and drain water areas are rendered as `class=lake`.  Since these types of objects are all flowing water, they should be grouped together into `class=river`, which allows for styles that render flowing water differently to do so on a consistent basis.

layers/water/water.sql

@@ -1,6 +1,7 @@
 CREATE OR REPLACE FUNCTION water_class(waterway text, water text, leisure text) RETURNS text AS
 $$
 SELECT CASE
+           WHEN water IN ('river', 'canal', 'stream', 'ditch', 'drain') THEN 'river'
            %%FIELD_MAPPING: class %%
            ELSE 'lake'
            END;