Validate originators from a Site/RegionModel in relation to a ModelRun #498
Description
Site/RegionModel originators should be validated against a ModelRun's performer field. This is a more meaningful validation than the current one of checking if an originator exists in the database. Furthermore, there doesn't appear to be any usage of the originator field in Site/RegionModels.
Reference: discussions in #497, PR removing the validators #499, and the PR that added the validators #424.