Add migration to create an index on load_new_guix_revision_job_events

To speed up the rendering of the index page.

sqitch/verify/index_on_load_new_guix_revision_job_events_job_id.sql

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