Add some initial Sqitch migrations

These are based on the state of the current manually managed database.

sqitch/verify/appschema.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:appschema on pg
+
+BEGIN;
+
+SELECT pg_catalog.has_schema_privilege('guix_data_service', 'usage');
+
+ROLLBACK;