Add a latest_build_status table

This will avoid many queries trying to figure out what the latest build status
is, which will hopefuly simplify queries as well as improving performance.

sqitch/verify/create_latest_build_status.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:create_latest_build_status on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;