Rationalise references to related objects within each entity type

[jackdos]

PAR objects reference other objects in a multiple places (Preservation Actions contain information about the tool they use, Business Rules contain information about the Preservation Actions they apply and the formats they apply to etc.), and we use two different mechanisms for doing this.

In some cases, (PreservationAction -> Tool), a full object is listed. This makes the reference type-safe and means that the full context of the object is available in its definition.

In other cases, (Business Rule -> FileFormat), just an ID is listed. This makes the object more compact (and arguably easier to read), but means that objects other than file formats can be listed in error and multiple API calls are needed to see the full context.

There are pros and cons of each method, and there may be alternatives that reach a better compromise than either of these two ways we currently have.

We should agree a single approach for recording these relationships/references.