Add internal utils for creating, updating, and deleting relationship

[gabestein]

Motivation

So developers have a way to easily create relationships on their own (currently only possible when creating a new pub)

Requirements

• Ability to create a relationship between two pubs
• Ability to update the value of a relationship between two pubs
• Ability to delete a relationship between two pubs
• Should only allow creation of relationships for which a relationship field already exists

Acceptance Criteria

• Function to create a pub and immediately add it as a relationship to a different pub in one transaction
• Function to create a relationship between two existing pubs, optionally with a specified value
• Function to update the value of a relationship field given a relatedPubId.
• Update the existing updatePub function to not be able to update relation fields