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/mapping.yaml

@@ -107,6 +107,10 @@ tables:
       - dock
       water:
       - river
+      - stream
+      - canal
+      - ditch
+      - drain
       - pond
       - basin
       - wastewater

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;

layers/water/water.yaml

@@ -25,7 +25,11 @@ layer:
     class:
       description: |
           All water polygons from [OpenStreetMapData](http://osmdata.openstreetmap.de/) have the class `ocean`.
-          Water bodies with the [`water=river`](http://wiki.openstreetmap.org/wiki/Tag:water=river) tag are classified as river.  Wet and dry docks
+          The water-covered areas of flowing water bodies with the [`water=river`](http://wiki.openstreetmap.org/wiki/Tag:water=river), 
+          [`water=canal`](http://wiki.openstreetmap.org/wiki/Tag:water=canal),
+          [`water=stream`](http://wiki.openstreetmap.org/wiki/Tag:water=stream),
+          [`water=ditch`](http://wiki.openstreetmap.org/wiki/Tag:water=ditch), or
+          [`water=drain`](http://wiki.openstreetmap.org/wiki/Tag:water=drain) tags are classified as river.  Wet and dry docks
           tagged [`waterway=dock`](http://wiki.openstreetmap.org/wiki/Tag:waterway=dock) are classified as a `dock`.
           Various minor waterbodies are classified as a `pond`.
           Swimming pools tagged [`leisure=swimming_pool`](https://wiki.openstreetmap.org/wiki/Tag:leisure=swimming_pool) are classified as a `swimming_pool`
@@ -34,7 +38,7 @@ layer:
         dock:
           waterway: 'dock'
         river:
-          water: 'river'
+          water: ['river', 'stream', 'canal', 'ditch', 'drain']
         pond:
           water: ['pond', 'basin', 'wastewater']
         lake: