Use YML definition for loose foreign keys

To avoid rails model preloading issues and EE/FOSS related problems, we need to move the loose foreign key configuration to a yaml file.

More context: !66889 (comment 735825940)

Steps:

• Introduce the YML file containing the raw loose FK definitions.
• Load the YML file and validate it.
• Update the Loose FK services to use these definitions rather than the actual AR models.
• Update the docs.