Add a new table guix_revision_lint_checkers

To associate a set of lint checkers with a specific revision. While there is
the association through the lint warnings, that only works for checkers where
there are lint warnings for that revision.

Therefore, to allow finding all the checkers for a specific revision, also
associate them directly with the revision.

sqitch/verify/guix_revision_lint_checkers.sql

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