Add support for county seats (capital=6) (#1401)

This PR adds support for `capital=6` in the place layer, which is used for tagging county seats in the US.  A county seat is the capital of a county.  In addition, I reorganized the code a bit to be simpler.

As noted in ZeLonewolf/openstreetmap-americana#384, we would like to render county seats along with state and national capitals, as this is typical styling on American-style maps.

Update with also with `capital=3` and `capital=5`

layers/place/capital.sql

@@ -2,8 +2,8 @@ CREATE OR REPLACE FUNCTION normalize_capital_level(capital text)
     RETURNS int AS
 $$
 SELECT CASE
-           WHEN capital IN ('yes', '2') THEN 2
-           WHEN capital = '4' THEN 4
+           WHEN capital = 'yes' THEN 2
+           WHEN capital IN ('2', '3', '4', '5', '6') THEN capital::int
            END;
 $$ LANGUAGE SQL IMMUTABLE
                 STRICT