Core_DAO - fire links_callback for all entities

[colemanw]

Overview

CRM_Core_DAO has a fallback getReferenceColumns for entities with no foreign key fields. That function was empty, so did not run the callbacks which are called by most other DAOs' getReferenceColumns.

Before

Fallback function did not call createReferenceColumns or fire links_callback

After

createReferenceColumns always called and links_callback always fired for any DAO::getReferenceColumns().

[civibot]

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[eileenmcnaughton]

OK - this feels like it would be safe because the other DAOS already do it - I think if there are no jenkins issues it's all good.

My worry was whether there could be any order issues - but Domain BAO already calls this hook & that would be one of the earliest to be intantiated