Sub-national routes in Canada and refactor national network list (#1446)

This PR defines the set of routes in Canada which are equivalent in national importance to the US Interstate Highway System, and are therefore appropriate to render at zoom 4. This creates a sane, connected highway network at this zoom level across the USA and Canada.

This adds two additional network types for Canadian highways, ca-provincial for provincial-level roads, and ca-provincial-arterial for "highest importance" roads that are not part of the Trans-Canada highway but should be regarded as equivalent for low-zoom rendering purposes.

Additionally, this extracts out the country-specific network checks to a separate function in order to define "equivalent top-level networks" by country, providing a place to add additional national definitions as they're added by contributors.

layers/transportation/update_transportation_merge.sql

@@ -331,9 +331,7 @@ BEGIN
         (highway = 'motorway'
             OR construction = 'motorway'
             -- Allow trunk roads that are part of a nation's most important route network to show at z4
-            OR highway = 'trunk' AND
-                network <> '' AND
-                network IN ('ca-transcanada','us-interstate')
+            OR (highway = 'trunk' AND osm_national_network(network))
         ) AND
         ST_Length(geometry) > 500;
 
@@ -356,8 +354,8 @@ BEGIN
     FROM osm_transportation_merge_linestring_gen_z5
     WHERE
         (update_id IS NULL OR id = update_id) AND
+        osm_national_network(network) AND
         -- Current view: national-importance motorways and trunks
-        network IN ('ca-transcanada','us-interstate') AND
         ST_Length(geometry) > 1000;
 END;
 $$ LANGUAGE plpgsql;