Support polling git repositories for new branches/revisions

This is mostly a workaround for the occasional problems with the guix-commits
mailing list, as it can break and then the data service doesn't learn about
new revisions until the problem is fixed.

I think it's still a generally good feature though, and allows deploying the
data service without it consuming emails to learn about new revisions, and is
a step towards integrating some kind of way of notifying the data service to
poll.

sqitch/deploy/git_repositories_poll_interval.sql

@@ -0,0 +1,8 @@
+-- Deploy guix-data-service:git_repositories_poll_interval to pg
+
+BEGIN;
+
+ALTER TABLE git_repositories
+  ADD COLUMN poll_interval INTEGER DEFAULT NULL;
+
+COMMIT;