Added support for a retry interval to monitors

If a check fails and retries are used you can now specify a specific value for that. So you can check faster if the site goes back up again.

db/patch-add-retry-interval-monitor.sql

@@ -0,0 +1,7 @@
+-- You should not modify if this have pushed to Github, unless it does serious wrong with the db.
+BEGIN TRANSACTION;
+
+ALTER TABLE monitor
+    ADD retry_interval INTEGER default 0 not null;
+
+COMMIT;