Add ability to update and delete pub field values, including relationships, to API

[gabestein]

Motivation

• So developers can work more granularly with fields in the API, including relationships

Requirements

• Ability to update an individual field value in a pub, including a relation field
• Ability to delete relation fields from a pub

Acceptance Criteria

• Uses utilities for updating relation fields in feat: add functions to interact with relationships #764.

Extra work

• Maybe: ability to update a related pub's relation fields? E.g. update a pub's title and also update the title of a related image in one go?

[3mcd]

@allisonking Do you think one endpoint should handle all of this? Or one endpoint per operation, kind of like @tefkah has each relationship-related function broken out?

[allisonking]

Since we get an editor state from the the editor rather than a diff, I think ideally everything is one upsert, otherwise we'd have to track "this pubfield was removed" and then call the remove endpoint. but I also imagine doing everything in one upsert might be more difficult?

[gabestein]

An open question: should we do this RESTfully, or is this complicated enough that we want an RPC-style structure

[allisonking]

Although now that I'm looking at feat: add functions to interact with relationships more, there's a test specifically to not allow updating related pub relations via the current updatePub function. I think this means the editor can update its own local pub, and then maybe call updatePub on each of its related pubs?