Add link_set for updating links on StandardEdition update [WHIT-2828]

[patrickpatrickpatrick]

What

• Add link_set to ConfigurableDocumentLinks
• Update patch_links in PublishingApi to call link_set (if presenter has method defined)

Why

A support ticket came in that raised an issue of associations persisting across the different types of News Article. This is because when a draft edition is updated, the PATCH endpoint for links is called. This creates a link_set of the links of the edition. The creation of the link_set means that even if subsequent PUT requests are made with a new set of links, the links associated with the link_set of the document will persist. Therefore we need to include empty array for each association that the document does not utilize. When we do this, the association will be cleared from the content item and so can't be accessed by the frontend rendering applications.

Implementation details

I did try and destructively remove the associations on the edition itself. The problem with this though is that an association can have a different name to how it is called on the edition. ministerial_roles is role_appointments, so without some of way of mapping the name change there wasn't a way to do this programmatically.

Personally, I quite like the non-destructive approach since it means the document can be switched back and forth and not need to be refilled in again.

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[ryanb-gds]

Unfortunately I don't think this is going to work, because while it's ok for news articles, other standard edition types may not allow all of the available association types as part of their links object, and this will cause Publishing API to reject the request