BaseLoader: Add 'origin_url' argument and remove 'prepare_origin_visit' method
Loaders only use prepare_origin_visit to set the origin and visit date,
so it is simpler to do it in BaseLoader.__init__.
Migrated from D7610 (view on Phabricator)
Staging instance, all changes can be removed at any time
Loaders only use prepare_origin_visit to set the origin and visit date,
so it is simpler to do it in BaseLoader.__init__.
Migrated from D7610 (view on Phabricator)