Move the `last_scheduled` ts from ListedOrigin to OriginVisitStatus

this timestamp being actually a loading-related value, it makes more sense to keep it in the OriginVisitStatus table.

Related to #2444.

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Migrated from D4881 (view on Phabricator)

sql/updates/24.sql

+insert into dbversion (version, release, description)
+       values (24, now(), 'Work In Progress');
+
+alter table origin_visit_stats add column last_scheduled timestamptz;
+comment on column origin_visit_stats.last_scheduled is 'Time when this origin was scheduled to be visited last';
+
+-- no need for a proper migration script of this last_schedules column: this
+-- have not been published or deployed; just drop it
+alter table listed_origins drop column last_scheduled;