Allow prioritising jobs from specific repositories

To help the QA data service prioritise branches over patches.

sqitch/deploy/git_repositories_job_priority.sql

@@ -0,0 +1,7 @@
+-- Deploy guix-data-service:git_repositories_job_priority to pg
+
+BEGIN;
+
+ALTER TABLE git_repositories ADD COLUMN job_priority INTEGER NOT NULL DEFAULT 0;
+
+COMMIT;