Convert Validation rules on Relationship object to a Trigger

[skosgi1]

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• Do you want to request a feature or report a bug?
  feature

• What is the current behavior?
  Currently there are validation rules on Relationship object, which would make sure that the Relationship's Related Contact and Contact are not changed.

• What is the expected behavior?
  Move the functionality from Validation rules to a Trigger.

• What is the motivation / use case for changing the behavior?

• Acceptance Criteria
  When a user edits "Contact" or "Related Contact" fields on a Relationship record it should not save the record and give an error message "Rather than change the Contacts in an existing Relationship, delete this Relationship and create a new one."

[skosgi1]

**lurch: add

[LurchTheButler]

Tracking W-034981